I think I want to sleep with an ex-fwb's sister
Why do you want to sleep with her?
I find her attractive and I'm very lonely.
You shouldn't sleep with your friend's sister just because you are lonely. You might ruin your relationship with your friend. You should see if things are ok first before making a rash decision.
I guess you are right. It's better to be honest and upfront then to ruin my friendship.
It really is the best way. I know it's hard to control feelings, but it's great to not act on these urges because you are lonely. How about trying out a dating app, or going out to a bar to meet others?