The title says it really:
Can you use the bodily substances (hair, blood, etc) of one twin to cast spells at arcane connection range and/or gain penetration multipliers on the other twin?
My character has an evil twin and I want to know if twins count as arcane connections to each other. I would love to say that I am planning to use it to cast nasty spells on my characters evil twin but truthfully it is more that I am afraid that things will go the other way. Since the evil twin is an NPC it is also something I would love to have clarity over before it becomes relevant in the game, as I would not want to spring a nasty surprise on the storyguide by casting AC range spells on my character's twin if the storyguide does not know that this is a possibility.
From a storyteller's point of view, there must be a two way AC to your evil twin! That is the purpose of evil twins, right? See The Dark Mirror, Dead Ringer and Jeonseolui gohyang!
I can't show you an Ars rule that absolutely makes it so, however. Maybe it is just the property of true evil twins, while the ACs between garden variety twins fade after a decade or two? So better talk to your troupe and your storyguide before using that AC.
EDIT: There is the Range Adelphixis in HoH:S p.122 Orphic Magic, which certainly reaches to your evil twin, if Evil Twin is a Story Flaw for your character. "In essence, the magus is using himself as the Arcane Connection required for the spell."
Ancient Magic, page 59 has the flaw Corrupt Fertility Ritual. How relevant that is to twins is another story, but its at least slightly similar?
There are definitely similarities as well as dissimilarities there. Thanks for pointing it out as I would not have noticed otherwise.
My character in the game knows her own date of birth as well as her twins birthname so sympathetic connections are as strong as they can be. I can only assume that my character's evil twin also knows their shared birthday.
However you cannot target someone through a sympathetic connection.
You make a strong case about the Adelphixis range OneShot. I assume that the existence of the Adelphixis range implies that targeting your own twin by using yourself as an arcane connection is not possible without access to orphic magic?
I never actually picked a story flaw for the evil twin. Another person in my group came up with the idea just by analyzing my character sheet and because she wanted to tell a story about how my character has an evil twin. However I would make the case that this is an instance of my character acquiring a new story flaw in play. Although it has never been put on my character sheet as a flaw it has the exact same function as a story flaw in that it gets my character to go out on stories. (I am totally fine with getting an extra story flaw in play).
Rather, Adelphixis can also be used to target your (ArM5 p.53) Enemies and (ArM5 p.56) Mentor.
Like me, your troupe may well feel, that Evil Twin would be still closer to your character and warrant the full AC in both directions.
That is my own gut feeling too and it will likely inform any house rule that my troupe makes, because I am sure that it will come up eventually.
But I am curious if there are anyone else who has experience with using yourself twins as an arcane connection in play and if there is a ruling somewhere in RAW that can serve as a base for my troupe to work with.
I think @OneShot's answer is excellent.
However,I do not think the evil twins arcane connection to the other, implies that shed hair and body parts from one make arcane connections to the other. The bond between the two is more spiritual than physical, so my gut feeling is you need the living twin in person to make an arcane connection.
The Story Flaw closest to Evil Twin that I have found is (RoP:TI p.89f) Tainted Offspring. And that leads to the question, whether - and if so, how long - one has an AC to one's child. Which then again leads to a specific Target only available with a Virtue: (ArM5 p.93) Bloodline.
Yep, typically a magus just can get and fix an AC to his child while it is still helpless in the cradle. But a Merinita can always target all his children, whereever they are, plus himself, by T: Bloodline. So this is also close to an AC - but not quite the same.
Thanks for the many good answers. I feel like I have a much better understanding of the topic now. I am glad to have an informed opinion on the topic before this will eventually have to be settled by and with my troupe.
I would mark the thread as solved but there are more than one of you who has provided a good answer so I would feel bad for all the ones that would be left out.