Future sister-in-law is a bridesmaidzilla

I have asked my partners sister to be a bridesmaid as I thought it was the right thing to do but to be honest I only ever wanted my own sisters as bridemaids. She is being a complete bridesmaidzilla and I don't know what to do. She invited herself wedding dress shopping when I just wanted to go with my Mum & my sisters, she hasn't liked any of the bridesmaid dresses I've picked (she's tried on more than I have and I am the bride!) she doesn't like any of the ideas I have for the wedding, she doesn't like the hair or makeup artists I've booked. She moans about literally everything until she gets her own way. There is now on ongoing joke that it's her wedding as she is taking over everything and it's really starting to ruin my bride to be experience as well as the experience for my sisters as their ideas are constantly rejected. They have even stopped having an input now as they know they will get shot down. I am so worried about the morning of the wedding as I just know she will make it all about her and moan about everything as well as getting in the way of special experiences and moments I would like share alone with my parents and family. I feel that everything I say or do is a problem for her and so I have started to not tell her about certain things I have done and then she finds out somehow and makes it very clear that she is not happy that she wasn't involved. She can be passive aggressive and I do not like confrontation so I don't know what to do.....I am at my witts end. My partner is aware of the situation and he agrees with me however he is worried that if we say anything she will fall out with us. Please help!