I think having lost someone dear to you and then having an opportunity to regain that person and rebuild that bond is bound to change a person on some level or another so I can see how she'd be more essential to him here in Marina.
"Sibling bonding sessions" sounds entirely inappropriate and incestuous! But assuming nothing's going on outside of cuddling, Jennifer probably wouldn't make to big a deal out of it besides thinking it was cute and take a picture! However, if it involves something more than that... Jennifer would do a double take, wonder if she was seeing things, and then probably ask if they were half-siblings and therefore not blood related! And blush the entire time she was asking her question.
She'd actually be conflicted because she does believe that love transcends all things so she'd be hard pressed to deny anyone that if such is what it was, but she'd also have that niggling feeling in the back of her head that says something's not quite right even if she can't put her finger on it because she's refusing to think of social standards.
And... on a different line of thought, out of all the characters in Marina, which one would you like to become best friends with either Adult Chall or younger Chall? Even though you didn't forth wall her, she's still a muse!