Is it normal to ask...?

So I have two friends that were dating for awhile, my best friend and a friend that is really close to me. They just recently broke up and I've been wanting to ask her out for many years now.

I care about my best friend but he's away to college so he's not really in touch with either me or her. Her brother is like a brother to me so we started thinking of eachother as siblings too. So we always introduced eachother to other friends as brother and sister.

Awhile before she started to date my best friend we made a deal to if we are single at the same time past the age of 18 (her and my parents didn't like the fact of us dating when we were younger) then we will give it a shot. I'm not sure if she will hold up to that deal or if she even remembers it. I'm going to ask her on a date at the right time weeks after she's done coping with this recent break up. Is this normal (after telling all our friends that we are basically adopted brother and sister) to even want to ask her on a date?