How to tell someone i dont want them to be my bridesmaid?

My mom insisted I have more bridesmaids than I originally wanted. I said ok fine and next thing I know my friend tina says " who's the other bm going to be" I dealt I had to ask her. But I hardly know her compaired to many of my friends, not to mention I feel like I was pinned into asking her. I do want another bridesmaid but I don't want it to be her. I don't want to ruin a friendship.