I'm going to sleep with my bff's brother.
Why do you want to sleep with your friend's brother?
I have feelings for him and he's hit on me many times before too.
There's nothing wrong with being attracted to your friend's sibling, but acting on those feelings and sleeping with him could lose you your friendship. Your friend might be so upset with you sleeping with their brother that they don't want to talk to you anymore. Think carefully before you decide if you really want to risk that.
That's fine by me. I'm only doing this because I'm attracted to him and he has a crush on me.
I guess I can't see what would be wrong about that. If you are both consenting people then it should be fine.