You have him for the companionship and the raising of a family, and his delinquent sister for sex, apparently.

I don't think it's weird to have feelings for someone without being sexually attracted to them. I'm not sure how your other friendships have been, but there's a few close friends that I admire very much, and admiration can feel like a crush even in the absence of sexual desire.

I think affection, like sexuality, is a spectrum rather than a binary thing.

The important part is whether you're both comfortable. And maybe what happens when either of you get into a serious relationship with someone else.