Sorry that you're feeling guilty and having to plan everything on your own. I hope you and FI don't argue too much about this. Don't get yourself too worked out over this. You should be honest with FI and let him know how you feel about and put himself in your position. maybe he'll get an idea of how you feel and what you are going through. in the end whatever you decide, as long as you are surrounded by people you love and care about, she'll be a memory only pictures bc you can totally ignore her on your day and enjoy yourself with the people who matter.
Like moemarsita said, being a BM comes with responsibilities and because she lives so far and has family obligations, it may difficult for to be an active member. If FI really wants her to be apart of your day, why don't you have her do a reading, then again it depends on what type of ceremony you are having.
Really, you do not have to make her a BM because FI only wants her to. Another thing FI can do, is make her stand with his GMs and make her a GroomsWoman. I've read an article about a male being a BridesMan. The entourage should be a decision both of you can agree on, but ultimately the decision is yours for your BMs and the same for FIs GMs.
One more thing, if having her in the entourage is important to him, put a male who is important in your life to be a GM. And although soon to be SIL is a BM, don't give her any responsibilities and just make sure FI does all the contacting with her to be sure she gets her dress, etc. I'm just suggesting here.
good luck!