For consideration- even though some make a big deal about Islam's inclusion, many rules are very christian oriented, so I looked up the seven grave sins of Islam, for discussion about how these would relate to the infernal and divine in game:
Note that number 2 has a significant impact on the game, especially the OOS.
Although here there are 70...
What makes up the Kaba'ir is a very complicated question that honestly is beyond the scope of a game text. There is essentially no consensus on a list beyond the idea that shirk al akbar (shirk is often separated into grades) is the gravest sin. There isn't a listing of them in the (classical Sunni) hadith literature or the Qur'an itself. Imam Abu Hamid Muhammad Al-Ghazali, who is already a great authority by game start, discusses some of the variance of opinion even from the Sahaba and Tabi'un in his famous Revival of the Religious Sciences:
Sins are divided into venial and mortal. There is much disagreement about them. Some say: There is no venial or mortal [sin], rather, every transgression of God's command is mortal sin. This is weak since God said: If you avoid the major sins forbidden to you, We will absolve you of your lesser misdeeds and admit you into a place of honour. . . and the Prophet said: `The five prayers and Friday [communal prayer] to the next, atone for what is between them, if major sins are avoided' and in another version, ' . . . are expiations for what [comes] between them except the mortal sins.' The Companions and Followers differed in [setting] the number of mortal sins. [It ranged] from four, to seven, to nine, to eleven or more. Ibn Mas'ud said: 'They are four.' Ibn 'Umar said: 'They are seven.' Abdullah b. 'Amr said: 'They are nine.' When Ibn 'Umar's statement, that their number is seven, reached Ibn 'Abbas, he would say: 'They are closer to seventy than seven.'
Imam al-Ghazali then, quite reasonably imho, sets aside the idea of constructing a definitive list altogether:
Major sin, then, is lexically vague and has no specific connotation, either lexicography or in law. This is due to the fact that 'major' and 'minor' are modifiers. Any sin is major in comparison to what is beneath it and minor in comparison to what is above it....To strive for a comprehensive definition or a definite figure is to strive for the impossible. For this is impossible except by hearing the Prophet of God say: 'By mortal sins I meant ten or five,' and list them. But as this is not reported...it becomes clear that no specific number was meant. How could one then aspire to set a number when the divine law does not?
al-Ghazali instead decides to provide general classes of sin that should be considered major:
Anything, then, which is in the way of perception of God is the gravest of mortal sin, the next being that which is destructive of human life, and the next is that which impedes the livelihood of the people. These are three stages.
al-Ghazali's ultimate position on major sin owes much to his studies on question of the Maqasid al-Shari'a - the Higher Objectives or Intents of the Law: the guiding principles which Muslim legal theorists considered to stand above law itself and were determined by attempts at close readings of the Qur'an and Sunnah. This field would be really brought into its own by an Andalusian scholar born exactly a hundred years after game start named Abu Ishaq Ibrahim al-Shatibi, but it had been a topic of study since at least Imam al-Tirmidhi wrote on maqasid in the 800s. I'd say the broadly Ghazalian position which could roughly be called the mainstream take in Sunni scholarship (as much as there is one at all) on this question at game start. Sadr al-Din Konevi, the son in law and major student of "Greatest Shaykh/Shaykh al-Akbar" Ibn al-Arabi who becomes the most major legal thinker and philosopher of Seljuk Rum in his own right, puts the opinion well when he says major sin is...
..that which poses a serious threat to human
life, the livelihood and prosperity of man, the progeny of families, the maintainence of religion, and the safeguarding of the faculty of reason.
Which again is a much more fluid way of considering the question. Now, this is far from "Islam's position on Major Sin" - hopefully I've gotten across that the idea of listing The Opinion (tm) is a fool's errand - but it is probably the one closest to a consensus among major Sunni usuliyyun (legal theorists) at the time that Ars Magica games tend to be played. Honestly, it might be wise to include a section talking about the multiplicity of opinions plus al-Ghazali's three classes of major sin in some future version of RoP: The Divine. @silveroak certainly did good to open the question.
Meh, there's a huge variety of period opinions here as well (you may now be detecting a pattern.) Internet search results don't tend to be representative of that diversity. Much of it is to do with the often not great translations used for the array of Arabic words employed in describing "magical" practice. Our friend al-Ghazali once again elaborates on what is roughly the mainstream Sunni take in 1220:
As for sorcery, if it contains unbelief it is a mortal sin; if not, its gravity depends upon the damage or good which results from it, such as loss of life, sickness or the like.
Unless it is explicitly pagan, the morality of "sorcery" depends on what you do with it, according to al-Ghazali. It's true, though, that jinn summoning tends to be one of the more sketchy kinds - the more intellectual connotations surrounding late antique theurgy as practiced by Neoplatonists like Iamblichus or talismanic magic are often more in tune with the inclinations of the ulema - but there is certainly ikhtilaf (can be translated as "scholarly difference of opinion concerning a religio-legal matter") on this question.
There's a 9/10 chance it would swiftly devolve into Islamicate Neoplatonist Criamonposting and Games from the Hikmat al-Ishraq doesn't have much ring to it when compared to Mr. Ferguson's wonderful program.
So I guess we'll wait till you propose a Tribunal book to Atlas, then. 
Our young Ottoman friend has done a fantastic job of answering this question, and I don't really have anything to add.
But I'm a historian, so of course I'm going to keep talking... 
I think that it is worth noting that when al-Ghazali states that what you do with sorcery is the most important issue, he is reflecting what has, from my reading, struck me as the Muslim majority view about magic.
This speaks to a much wider point about Islam, which is that the intention of the individual, whatever they do, is key, and the general Islamic view is that God recognises that we're human and we're going to mess up from time to time. A Muslim might not intend to drink alcohol, for example, but what happens if someone spikes their drink? Should this be held against them? Most Muslim scholars would say no, because the intention of the Muslim was to avoid alcohol. Likewise, a doctor might intend to cure a patient with a new medical technique, but if it all goes horribly wrong and his patient's health gets worse, the intention of the doctor is still approved of.
So by the same token, if a maga uses her skills for good ends, such as curing the sick or defending the powerless, even if the magic involves summoning jinn, her actions will still be approved of.
The issue with magic, of course, is that it offers such an easy path to greatness, and makes a magus feel like they are the only power in their world. Let's end with a quotation from Qur'an 2:102...
"It was not Solomon who disbelieved, but the devils disbelieved, teaching people magic and that which was revealed to the two angels at Babylon, Harut and Marut. But the two angels do not teach anyone unless they say, 'We are a trial, so do not disbelieve.'"
To be fair there is also some indication that the djinn may have been considered deities before the rise of Islam, similar to the way the Titans were possibly echoes of Babylonian and Sumerian gods, so that might have had something to do with the reasons that dealing with djinn would be considered sinful as well.
And even if that's not true in the real world™, it's good enough for Mythic MA.
There's certainly some evidence of the former, Amira El-Zein's wonderful Islam, Arabs, and Intelligent World of the Jinn goes into it, but it's not so much dealings with the jinn that were ever problematic. There's a whole chapter of the Qur'an about a party of jinn who visit with the Prophet and convert to Islam. Jinn marriages were (technically, are) legal according to the major opinions of three of the four currently dominant Sunni madhabs, jinn scholars are in the isnads of some hadith (albeit ones that typically also have multiple alternate chains), Muslim jinn preachers console grieving parents who lost children and exhort crowds of believers at night prayers in narrations, etc. Close dealings with the jinn, as close as husband and wife, weren't really the issue.
The problem was always more with what jinn based magical dealings entailed in the culture. Soothsaying using the jinn was a big issue, since it was believed that the jinn snuck up to the ramparts of heaven and spied upon the doings of celestial beings then conveyed creative interpretations of what snippets they heard to their human partners. Muslims believed that upon the advent of the final revelation, the angels stepped up their defense and loosed flaming bolts that struck at the spying jinn and prevented them from learning anything of the workings of heaven. It's got some great imagery in the Qur'an, when the party of jinn are talking to the Prophet.
˹Earlier˺ we tried to reach heaven ˹for news˺, only to find it filled with stern guards and shooting stars. We used to take up positions there for eavesdropping, but whoever dares eavesdrop now will find a flare lying in wait for them. Now, we have no clue whether evil is intended for those on earth, or their Lord intends for them what is right. - Chapter of the Jinn [72:8-10]
So any soothsaying now would just be wholly lies - later scholars by the 1200s would point out that jinn could just also be using their abilities to travel and gather news to bring information, which connects to the more ambivalent view. It also ties into discouragements around casting lots and bird omens, although prognostication in a more general sense (such as - later - Greek astrology, which was seen as a science ofc) was obviously not verboten. Jinn allies in "magical practice" were also regularly directed at the wells or herds of rival tribes during inter-tribal warfare, even to the point where it was believed sahirs targeted the infant children of rivals and broke customary laws of war (the Quraysh and their fellows were old hat at this, though, one of the last major wars before the Prophet's day - he actually "fought" in it as a youth - was literally called The Sacrilegious War for having been fought during holy months.) Jinn possession was a common association and the accompanying madness a feared affliction. Although it could be argued that using a jinn as a weapon of war isn't really any worse than usual soldiering or that astrology and jinn soothsaying were the same, the point remains that the older traditions connected to "magical" practice with the jinn weren't exactly wholesome from the new Muslim perspective for reasons beyond what the jinn were themselves. The positive associations of healing or estatic inspiration kept most scholars at the tentative "well, it depends on what you do with it" but astrology or talismanic writings or numerology didn't have this mostly negative historical baggage from the Muslim view which typically made them easier sells.
This is perfectly put and I think a good guideline in general to keep in mind when worldbuilding around Islam in games. It's not for nothing that the first hadith mentioned in Al-Nawawi's Forty, Riyad al-Saliheen, and many other classic collections is the famous "إِنَّمَا الأَعْمَالُ بِالنِّيَّةِ" or "Actions are by their intentions."