I want to hook up with an old friends younger sister
Why would you hook up with your friend's sister?
I think she's cute.
You should respect your friendship and not pursue their sibling for a relationship.  You may ruin your friendship if anything goes wrong with the younger sibling.
Why? I just want to fuck her.
It's still not a wise thing to do. All I'm saying is that it could ruin your friendship and you probably shouldn't pursue her anymore.