I'm going to date my best friend's sister
What is it about your best friend's sister that you want to date her for?
She is so cute and I'd like to get to know her better.
You have a right to your attraction and every right to make a move. Be warned, though, she is your best friends sister. It could make things weird.
I don't care. I just want to do her and she is going to do me.
I don't know about 'doing' her, I just wanted to give a little warning about the situation.