Okies. (A later post I made suggested Antoine would go back and accompany Isolde if she came by mundane means, but I'm sure he can be doing something else instead).
Or he could have gone back and been too late.
"Okay, then the first thing we need to do is find out where the smaller boats are, the dinghies or whatever they are called. If we wait for shore they are certain to keep an eye out. Also make sure your sister is with us, that is not a conversation we need to be having at the last moment."
Yanni is sure his sister Yenni is just as concerned about their uncle as he is. He was too old and got left behind. He will go talk to her in private, and returns confirming she is down with your plan.
"Right now we pretty much have freedom of the ship, we need to find the dinghy, then meet up after dinner." Does anyone have private rooms and what phase is the moon?
The captain decides to put the siblings in one cabin together, no guard. Donna is given a private room.
Whatever you are planning, just before leaving her room, Donna hears a knock on her door.
The captain is outside, a bit drunk.
[color=green]Donna my dear, I brought a bottle of wine. Ha ve a drink with me. His voice has a heavy suggestion of amorous intention.
"Captain, please come inside, make yourself at home, I just need to make a quick trip to the head then I will join you."
If he does as she suggests Donna will slip outside and lock her own cabin from the outside before meeting up with the others.
Well played 
Yes indeed, the captain had a lock put on your door, an impediment you would have discovered had you tried to slip out as you planned. As it is, you manage to confuse the captain with your confusing excuse (there is no "head", this is the age of the chamberpot).
So you lock the captain in, and he is to drunk to do anything about it. He falls asleep waiting for you. You realize that there will likely be a similar lock on the siblings' door.
I also need a stealth or guile roll. the deck is bustling with night crew as they track and pace the merchant vessel (mentioned earlier). They plan to strike at dawn, 6 hours or so from now.
First roll http://invisiblecastle.com/roller/view/4605571/ is a 1 (don't ask about the plus 13, should be +8 for guile), second http://invisiblecastle.com/roller/view/4605572/ is a 4, total of 16 on guile.
She will look for locked doors in the cabin area, as this is likely where the siblings are being held. What kind of locks are we talking about? Key locks or simple latches that can only be operated from the outside?
Excellent roll. You encounter pirates busy working and they uy whatever excuse you give them. Role play a good excuse comment or two and I will help you out even more down the line. As for now, I'll grant you the door is latched from the outside by a board that slides through loops, a makeshift piece of junk improvised when you each were put in private rooms.
"Captain wants me to get that Yenni girl, says he hasn't had two at once before. Don't know as I believe him."
"Sorry, didn't mean to be in the way, came up on deck because I was feeling a bit queasy."
"Why am I out here? I heard sailors were strong, and good with their hands, but this is really public, where is the dinghy? Can you meet me there about an hour before daybreak?"
You tell one guy about the twofer idea and he directs you to the proper cabin. Then the guy you ask to meet you at the ship's boat (I think dinghy is an anachronistic term, and I am not sure medieval ships had extra boats, but we will say this one does). He directs you to a skiff they have in tow and says to meet him in an hour and he'll bring the wine.
Get the two siblings from their rooms, then head towards the skiff "we need to hurry, someone will be along before too long. If he is there I'll distract him while the two of you jump him."
You three make it to the skiff without encounter. It is in tow, so you will have to jump in the water and swim over. No problem for Yenni, she is a strong swimmer. But Yanni the doughboy can't swim. Can Donna?
You have a half hour before the guy with he wine shows up to meet Donna.
How alluring and seductive is she? Doing some femme fatal acting here, need to get a grip on the visual.
Donna can't swim either. She'll have Yenni go down first and try to bring the skiff closer, while she and Yanni climb down to board it.
Give me an Athletics roll for Donna. Yanni has no score but has +2 Dex. Posting from phone, so fiat results are that he fails to make a graceful jump. He does land in the skiff, hard, and suffers Light Injury. And he makes a lot of noise, attracting attention.
Donna has to move fast!
Athletics is 2 (grace), dex is 5, roll was a 2 (I'm assuming you don't need proof of that roll), total 9 unless it is for being gracefull, then a 10.
That"ll do it 
Yenni cuts the line and is quick to get to the oar. She shout at her brother who struggles the best he can. The skiff moves away from the ship. Pirates start throwing things at you. One guy jups in the water and starts swiming after you.
Which way to go? What to do?
Since they have seen us, head for the ship they are stalking, once we get beyond the reach of what they can throw things. Hopefully we can row the skiff faster than the one pirate can swim, otherwise Yenni and I will attempt to conk his noggin with the oars when he is in reach.
Yanni the Doughboy is struggling at his oar and you are not making goos speed at all. The swiming pirate does indeed catch up, and is climbing over with a dagger clenched between his teeth. Yenni is non-Combatant (because her uncle taughter that girls don't fight), and Yanni is, well, he can give it a try
Pirate: Dex +1, Quick +1, Str +1, Brawl 4 (dagger)
Yanni: Dex +2, Quick -1, Str -2, Brawl 0
I will let you narrate it and roll everything out. 
Donna and the pirate go almost at the same time (both have init total of 7), but as the pirate is still clearing the side of the ship Donna has a slight advantage and wails away with her makeshift bludgeon. She scores a hit (attk 17 vs def 9- total dmg of 10, uncertain of soak but assuming around 4 he takes a medium wound), and he stabs back at her with blood trailing down his face (-3 from wound, attk total:8, donna's def total:9) and Donna blocks the blow. Yanni, realizing this man intends to assault his sister even if he never has been in a fight before, swings at his face and would have connected had he not flinched when he saw the dagger turn his direction. Yanni follows up quickly, this time connecting (10 attack score versus 7 def), but completely ineffectually. The pirate assesses that Donna is the greater threat and strikes at her again, giving her a cut on her chest (3 attk adavantage, 7 dmg, 0 soak, donna now has a medium wound as well), before donna strikes back (5 dmg, 4 soak, light wound) less effectively than before. Yanni, undamaged, again swings at the pirate, scoring a hit (7 attk advantage) on the injured pirate, inflicting a light wound. Yenni meanwhile has been putting distance between us and the pirate ship, and the pirate again attacks Donna, failing to connect. Donna strikes again at the pirate, hitting him again, again giving him a light injury.
The combat pauses for a second as the pirate assesses. If he returns, he was beaten by a woman and this... pastry chef. If he stays he is likely to be taken prisoner when the skiff reaches the ship, and he and this woman will likely both be worn out and badly injured. He should probably start worrying about the boy, who being uninjured is almost a match for him now, but that leaves the woman- he is effectively outnumbered, but retreat will bring only humiliation...