David loves his sister Poly a lot. In many was that aren't acceptable in polite company. But in the quiet of home, when no one's watching...he lets his raw affection loose. It's true that he loves her in much more lewd ways too, but the girl is special to him. Every kiss, every touch, all of it brought on by his deep love for his little sister.
So when he pulls her into his lap and hugs her, it's not just because he wants to pull her clothes off...even if they will probably get to that eventually.