I'm going to sleep with my brothers girlfriends sister
Why do you want to sleep with her?
I've been in love with her since I was a kid
Even if you've had such strong feelings for so long, you should restrain yourself. You are not free to sleep with whomever you want as this will cause a rift in your family.
Haha. I don't care. I'm going to ask her out.
I understand that you like this girl, but sleeping with a your brother's girlfriend's sister is not right. All of you are too close in your relationships and tying everyone together through sexual relationships will make things awkward.