I am in the exact same boat. My future sister-in-law is not one of my favorite people in the world. She is just not an all around nice person. Early in our relationship, my now finace, told me he would like to have his sister stand up in our wedding. We recently got engaged and he has already told her so will be standing up in the wedding as one of my bridesmaids. I guess my advice is if she means that much to your fiance, why not include her. It has already brought me and my future sister-in-law closer. Just let her know how important this day is to you and your fiance and what type of behavior is expected from the wedding party on that day. Best of Luck!

I also have a reply about the cousin with ADHD. I agree that you should talk to your fiance about why he doesn't want her as a junior bridesmaid. I would also suggest if you do not have her as your junior bridesmaid, find some other tasks she will be able to help you with. Like help with favors, decorate the church, and hand out programs on the day of your wedding.