Mums Wedding

So talking to my mum today - it sounds like the wedding is getting bigger and bigger, and she said the that bridesmaids are gonna be me and shenee (Her fiancee's 15 year old daughter)
I feel kinda weird about that - like is my mum only having her as a bridesmaid to keep Keith happy or does she want her as her bridesmaid - and if she wants her as her bridesmaid - WHY? she isnt her daughter, Im her daughter!
Why cant Shenee do something else in the wedding, why does she have to share my job with my mother?

Am i just being selfish in my thinking? am i over reacting? maybe i dunno. maybe i am being selfish and mean - but it kinda feels like im being abandoned by my mother and now have to fight with Shenee for my own mothers love.

I always thought bridesmaid was an important job and you chose a bridesmaid cos they were your closest friend - not just cos they are your fiances daughter.