@Laybyloitering I am sorry your step daughter is doing this. I think you are within your rights to stop her being a bridesmaid BUT I think in your shoes I would not.
I'd talk to her about how hurt you feel. Explain you understand she is in a difficult position (if you don't see it I expect someone else can explain). You are not the other women, you did not take her dad from her mum, but maybe, in some way, someone (her or her mum) feels your getting together with him stopped them getting back together, etc, ( I am sure it did not ).
So maybe she feels conflicted. Her mum is not helping at all. So you can be the bigger person here, if you want to.
Explain how you feel to your dsd (to find out she has been bad mouthing you) - please be sure she has and it is not ex wife stiring it up!
And yet despite all that bad mouthing you still want your step daughter to be your bridesmaid - if you do.
Do things with her, dress fittings etc, and do them separately with other bridesmaids (one to one) if there are others and if you feel your DSD may jeopardize anything.
Just for the record I had a 14 year bridesmaid (my God daughter) she was stunning and taller than me! She wanted to wear a different dress to the others and she wanted to wear heels despite me thinking it was not advisable, as it made me look short (I am)!
We compromised, she wore the signature colour and heels (even though the dress she chose was actually nicer than the one I chose!!!)
Anyway, I'd turn this into a charm offensive because the future is a bigger place than your wedding day. One day she may make you a grandma, and one day she may see the real you, the lovely, kind, caring, scrumptious you, and if I were you, I'd be holding out for that day and allow her some petty mean words.
(But I am a big old softie!)
XXXXX Enjoy your day, whatever happens, and remember it is all a marathon not a sprint. 