Genis, for the most part, trusts his sister. And despite the fact that she does have some sort of ulterior motives, something that very frequently he'd be able to tell, he doesn't this time. It's still weird, and he's not going to admit that Raine's making the most sense of this, but it's not going to trip his "hey wait something is really wrong here" sensors. Granted, he doesn't have the trigger issues with Mum that Raine has, so... he just shrugs it off, perhaps thankfully for Raine.

At least except for her reply. "You're... welcome, I guess?" he says, noticing her "strange" reply. Honestly he can't see how he's helping her out.

But of course, it's not going to be as simple as just celebrating a simple birthday. It's of course going to be more complicated. But Genis' eyes should be in a state of focus where Raine should be able to tell that, yes, Genis does know Bakura. "Yeah, of course. I actually thought he was a half-elf at first. He's kind of mysterious, but he seems nice."