So, I'm going to disagree a bit with others, I think. I keep re-reading your OP, and there seems like a lot of underlying issues here. I would be amazingly upset if any future IL of my aired all this dirty laundry online, including copying and pasting an entire email. You've made snarky comments about the length of time she and your brother have been dating, about her mental health, and from the very beginning seemed unwilling/unhappy about being a bridesmaid.
I don't blame her for being disappointed that you backed out now, instead of just declining in the beginning. She's probably upset and doesn't understand in general why you would say yes if you didn't want to do it. I do think you did the right thing to back out, because it seems like it would have just gotten worse. It's not your wedding, it's hers and your bother's, and if you can't be there for them fully, then it's best not to try to do something halfway. You and the rest of your family may not agree with everything they are planning to do, but that doesn't mean that they don't have the right to plan their wedding the way they want to. Regardless of who is paying. (I feel very strongly that if you give the gift of paying for a wedding, it is a
gift, not a right to control. If I give someone a birthday present I don't then have the right to tell them how to use it.)
I just think there is way more going on here than the dress issue, or the no-kids issue, or anything that relates to a detail in the wedding itself.
I'm sorry if this is overly harsh, but for some reason this whole thread really struck a chord with me. Maybe because I'm sensitive to the whole "they haven't been dating long enough" sentiment. My husband and I got engaged 5 months after we met, and that was 12 years ago. Maybe it's because I'm 12 weeks pregnant, with no end yet in sight of morning sickness and generally feeling like a disgusting, nasty blob, yet would gladly order and alter the bejeezus out of any dress if my brother's fiance were planning a frills wedding and wanted bridesmaids, no matter how fugly the dress may or may not be on me.
Or maybe it's just the whole pregnant/blobby/grouchy/hormonal bit in me coming out.
If that's the case, I apologize. But I feel sorry for your FSIL, for the fact that you are sharing all this online, and for the fact that you seem to think very poorly of her. I do understand the need to vent. I guess this just seems like too much information in too public of a place, that it feels meaner than a normal vent to me. But, I'm hormonal, so do take all this with a grain of salt.