How to/can I unask a bridesmaid?

I'm having my sister-in-law and my fiance's sister as bridesmaids in our wedding. I've asked an old friend, but now I'm regretting it for a few reasons. My fiance has decided not to ask a third groomsmen, she's planning a wedding around the same time I'm planning mine, I didn't ask other friends to be in my wedding (they will be hurt because I was in their weddings), and she hasn't asked me to be a bridesmaid in her wedding which to me indicates we aren't as close as I originally thought. Is it a really big deal to unask someone? I don't think she will be too upset, but weddings can bring out the crazy in everybody and I don't want to lose her as a friend. I just think it would be better to keep my party just family. Help!