I've been lurking around the forums since my brother got engaged, and a question I often see come up is "what do I do with the sister of the groom?" Party line around here will tell you to see if your future husband is willing to have her as a groomswoman, and after my experience so far as a bridesmaid in my brother's wedding, I just want to endorse that.
I have no beef with my FSIL. She's a really nice woman and my brother loves her a lot so I'm excited that they are getting married. But I don't know her very well. I had only met her maybe a dozen times when they got engaged. She asked me to stand on her side as a bridesmaid and I accepted. I was excited to get to know her a bit better, to plan a shower for her with the other bridesmaids and get to know her friends and sister.
I have since figured out that I am an "obligation bridesmaid" and that my FSIL didn't really ask me as a way of getting to know me. I was left out of planning the bridal shower despite asking the other bridesmaids many times what I could do to help. I was made to feel like I was lucky I was even invited. I wasn't included in dress shopping and was just told what to buy without getting to voice an opinion. I feel like I am not wanted in the bridal party and it hurts.
If I'd been allowed to stand on my brother's side, I KNOW that my other brother (the best man) would have loved planning the bachelor party with me, and I would have been welcome to come along. I'd have had some kind of say in what I wore. I'd have felt like part of the wedding party.
Please brides, if you aren't close to your FSIL, let her stand on her brother's side. That way if you or your bridesmaids don't want her involved in any of the plans, she won't feel slighted when she's not included. And she'll get to have a lot of fun and make some wonderful memories with her brother.