Congratulations, I do believe your wedding your rules but also it is uniting two families. Also agree with not letting MIL get upper hand as it lays down precedent for future so not much help really.
My Mum when we told her we were getting engaged turned immediately to my sister and said "ooh you are going to be a bridesmaid." Not even sure my sister wanted to be a BM or even understood what one was but there you go.
I wasn't planning on any grown up BM as this point, it then escalated into best friend as Chief BM, sister as BM, Best Mans young daughter as BM, nephew x 2 (young) as page boys.
All of these people were then at my Mum and Dads house whilst I was getting ready and wanted to some quiet time with my Mum and Dad.
I ended up helping get Mum ready and looking after all these other people as 3 were young and my Dsis has learning difficulties. Poor Chief BM didn't know what to do to help out and there were so many people milling around. My Dad ended up fastening me into my corset and helping me get ready as everyone else was doing their own thing. .
Anyway, that is beside the point, just needed a rant. The point I wanted to make is that my BF who was Chief Bridesmaid had her fella with her who only knew her, myself and the Groom. He was then sat with a group of strangers and looked a little forlorn throughout the day. I am just wondering if your SIL is still young if she has a partner at the time that could be newish they may feel left out and it may be better if she was not a bridesmaid.
Some sort of role might be nice but it is still a while away. You can always delay your decision on whether to have her as maid/usher/reader etc after sounding her out discreetly and getting to know her a bit better over the next 18 months.
Good luck, oh we had Best Man and FOTB as witnesses/signing the register and bought them both a nice pen to do it with. My Godmother did a reading and my Godfather said Grace before the meal. Bridesmaids got a necklace to wear on the day, Mums got flowers, and Page boys got Action Man type things.
We also did speeches before the meal when we were all sat down so that DH, BM and my Dad could get it over with and actually manage to eat their meal without being worried sick with nerves. Fairly traditional overall but we twisted things round to make them work for us.
Hope it all goes well.