Please help - Rel'p with neice ended by her mother not invited to my Birthday party

I have been a mentor to a young girl for 10 years, since she was 4. We are the best of friends. (It's an Australian equivalent to Big Brother/Big Sister). My relationship with her mother has always been challenging ~ she is very reactive & emotional, flies off the handle, takes things personally, very intense. I made the decision NOT to invite her to my 50th birthday party. For once, I was thinking about me....my rationalization was that I needed to be able to relax at my own party and when...