My cousin (let's call her Emma) and I are a year apart in age and have been very close our whole lives. Her sister (let's call her Alice), however, is 4 years older than I am and we are not close at all. She's "on the spectrum" and it is very hard to relate to her. Emma is beyond thrilled that I'm engaged and excitedly asked "Am I going to be a bridesmaid?!". It caught me off guard, but I said yes and I am happy to have her (she is also a concert pianist and has offered to play the piano as I walk down the aisle). The trouble is I really rather not ask Alice to be a BM as well. I think she'll feel very uncomfortable being in the spotlight that day, posing for pictures, planning the shower etc. and she doesn't know any of my friends (or other BMs for that matter).
Am I obligated to ask her to be a bridesmaid? My mother insists I must because my aunt will be offended, but I've already asked Emma and she ultimately I don't need to, and that Alice probably wouldn't want to be one anyway.
What do I do!?