Improper Wedding Attire 😭

I am getting married NEXT WEDNESDAY!!! We are getting married with just parents in a national park. We have asked for no white or black dresses, no loud patterns, and to try to be formal and somewhere near requested colors (fall colors, burgundy, wine, deep purples and blues). My FMIL is impossible to get ahold of but her partner (fiancés step dad) is pretty easy. He explicitly told the stepfather to make sure FMIL does not wear black or white at the minimum (she is a narcissist, I could go on for hours as to why and at the last family wedding she wore a skimpy homecoming looking dress). Fiance also texted her just in case she actually saw her phone. She showed up as a suggested Snapchat friend and I decided to stalk a bit… she had posted her trying on stuff yesterday. She was wearing a solid black dress with 3/4 of her tits hanging out and there were several white dresses behind her. I just want to scream. I didn’t even want her there, he hardly wanted her there, I hardly even wanted any parents there just to avoid drama but he is a bit of a pushover and couldn’t say no… she always starts something when the event isn’t about her and I fear she will try to do the same. She did it at the other wedding and at his sister’s FUNERAL. He claims if she tries anything, he is finally cutting her out but I don’t know if she actually will and I don’t know if he understands wearing what we explicitly asked her to not wear counts as trying to start something. And I am afraid that if she does anything out of line I might lose it because of how stressful planning all of this has already been.