This is a lot of angst over not very much. You're hostile to her - she has doubtless picked up on this however hard you might think you try to hide it - and she is retaliating by the low blow of bringing children into the mix. Not least, having bridesmaids as a mature bride seems pretty childish to me.
She already knows she's hit a nerve. Your FiL has told you that her bridesmaids are her business, therefore if he knows you're disgruntled about it, she soon will. In your shoes that would be the sum total of any satisfaction I was willing to give her.
I'm sick of the constant, tedious cajoling of women to be kind, to be nice, to put their own feelings last, and to 'be the bigger person'. It's gendered, and it's infuriating, and it's advice I very rarely give. However, I think from the details you've given, this situation is an exception. You say she thrives on creating division. If she really does want to use her wedding to do this, deny her that satisfaction. Attend with your daughters (dressed as guests, not additional bridesmaids), hold your head up high, be gracious and, outwardly at least, pleased for the happy couple. If she's of the disposition you say she is then this will irritate her far more than any equally passive-aggressive grand gesture. And what's in this for you? You get to keep the moral high ground and the knowledge that, if she's as divisive as this and willing to use her own wedding to indulge it, then the marriage likely won't last.
Once the wedding is over then consider distancing yourself and your daughters and putting in place firm boundaries to prevent any divisions being created between them. Nurture your relationship with MiL instead. But don't play into FiL's fiancee's hands by creating issues around the wedding. This will only give her the last laugh, that I can pretty much guarantee.