@NSMS,
@Alexander89 - spotted a possible conflict. Samantha being 17 unless she joins up after October 17, 2017 may be problematic, as anything else makes her older than Euphemia. The numbering of the princes and princesses is determined by birth order, at least according to the wiki. Euphemia is the Third Princess, with Cornelia as the second and Guinevere the first. She could be the fourth Princess in that case, as Carine ne Britannia is the Fifth and is 15. (though Nunnally is presumed to be the Fourth, but this is never confirmed, so she doesn't have to be)
Though then there's the introduction of Marybell mel Britannia, who is 17 at the time of Oz the Reflection and is somehow the 88th Princess, which I'd attribute to the writers of the spin-off not actually thinking about things, much like the whole Euro Britannia thing not making much sense. I guess you could just make the order of succession more complicated and just roll with there being some kind of weird, arcane rules that are never explained (maybe having to do with the order in which the particular wife of the Emperor was married to him, his favor, noble ranks, and things liek that - Charles does apparently have 108 wives) to get around it. Still, I thought it necessary to point out.
So, what will Lulu think when he finds out one of his siblings became a GSP?
It probably depends on what his own memories of her are. She's his age, so he probably would have met her before, though it's possible her mother was one of the ones who hated Marianne and so didn't allow her children to associate with him and Nunnally and he might not have much in the way of memories of her. Either way though, he'll be able to empathize in spades with her story about finding out that Britannia is evil and being betrayed by her parents. Lelouch damn well knows that their father is an asshole and that many of his wives are evil harpies.
That will be entirely up to Alexander and the dice. Though do note she has decent social stats- she might well be able to avoid lying directly when telling him.
She'd have to avoid telling him anything for him to not detect a deception.
Her dice pool for a Manipulation without Excellencies is four dice at best. Lelouch's Dodge MDV is 6, so any social attack she makes will require Excellency use to even have a chance of success, and he can use Excellencies too. Not to mention that his Dual Gaze Paradox gives him Essence sight so that he'll see that she's doing something with Essence, and he would likely recognize the Ebon Dragon Excellency as he'll have seen others do it by then since she's the last arrival. Then there's Lelouch's own dice pool to pierce through deceptions and notice someone is lying - he's got a base pool of five, two more added due to Seyrun, and three automatic successes due to Witness to Darkness.
Her best bet would be to get him to not ask somehow, or just outright tell him that her mission isn't something she can talk about (though that would arouse suspicions in Lelouch, and she'd probably know it) and hope that Lelouch doesn't somehow social attack her into talking.
By the by, you should give Samantha a dot in the Nobles specialty for Socialize, as even if she was sheltered there's no way she wouldn't have been educated in that.
I suspect Lelouch will be getting a message from TED telling him to not let Samantha and Isla meet. Then again, it could totally work if TED let Samantha in on the gag and let her act as Isla's handler.
I think Samantha would probably pity Isla as she would know the girl is being betrayed, but probably not tell her anything. It would be best not to have them in a Coven together given Samantha's Compassion likely causing her a lot of grief about it, though.