
IS SHE FUCKING HER BROTHER!??????

Fontaine? No. She's using the fact that he knows he wants to have sex with her to her advantage by emotionally manipulating.
Dion doesn't want her physically, like Fontaine, he wants her emotionally. She doesn't have to give him sex to please him. Just her love. It seems like they're at a point where he'll do what she asks in the hopes that he'll earn a place in her heart. It's not necessarily romantic love he wants, nor familial love. He just wants her to want him. The love she had for her brother that made her cry when she lost him, he wants that level of whole hearted attention and affection from her so he can see those emotions from her again.
But no, they ain't fucking.
Please why is he getting a redemption ark