My (32F) future stepdaughter (12F) destroyed my wedding dress a week before the wedding. I'm torn between giving her another chance or calling off the whole thing.

I am beyond frustrated right now and am on the verge of giving my fianc?, Alan (39M) an ultimatum that I really don't want to give.
Alan and I have been dating for about 3 years and have been engaged for a year, with our wedding date in a week. He told me on our first date he was a package deal; his daughter, Rose, is his whole world. I was perfectly okay with it and we started hanging out together. Alan's ex, Carrie (38F) is still in the picture as she and Alan are coparenting. Carrie doesn't like me one bit. Alan told me that he and Carrie had been high school/college sweethearts and she had always held onto hope that he'd come back to her. But Alan also confided in me that Carrie wasn't a great partner. She adores Rose. But she treated Alan badly during their entire relationship. You name it, she did it; emotional abuse, gaslighting, financial abuse, belittling, etc. He finally got the courage to walk away from her when he discovered she was having an affair. Rose was maybe 6 when Alan learned about the affair. Unfortunately, the courts still granted Carrie 50/50 custody, despite the amount of evidence against her.
Rose and I have had a bit of a strained relationship. I understand it, having grown up in a divorced home that both parents remarried. She's a preteen and has some conflicted thoughts about her dad marrying a new woman instead of going back to her mother. But she's been really haughty and nasty to me. She makes cutting comments and will respond with "You're not my mother" when I ask her to stop. Alan has tried to set down ground rules but she flippantly tells him that "OP's not going to last." I put up with it because I love Alan and, sometimes, Rose behaves herself.
Seriously, she's not a horrible kid. She gets good grades in school, she's in the youth symphony and takes dance classes. She's really popular, bubbly and friendly when she isn't being mean to me. It's like Jekyll and Hyde. Alan and I have talked about it constantly and have even involved Carrie but Carrie just brushes it off, telling me to mind my own beeswax. We have tried therapy, groundings and everything else but nothing works.
That brings us to tonight. A couple of weeks ago, alternations on my wedding dress was finished. I was really excited. My late grandma (who I was really close to) made her own wedding dress after seeing photos of Grace Kelly and many women in our family have worn it to their weddings or have had their own dresses designed after it. As I'm taller than my grandma, I had mine made. It's really special to me. I have the dress saved in my closet, tucked in a garment bag.
Alan and I don't live together. However, my apartment is close to one of Rose's friends so we have an arrangement that on some days Alan has custody, Rose will hang out at her friend's house and then come over for dinner with Alan and me. After we watch a movie or play Nintendo Switch, Alan will take Rose to his place. Today was one of those days. Alan was running late from work and called to let me know. Rose arrived and everything was normal. I stepped into the kitchen to finish preparing dinner, leaving Rose alone.
If I had any inkling of what was about to happen, I would've done something different.
Dinner was ready and I went to get Rose. I passed my bedroom door and something about the way the sliding closet door was positioned made me want to check.
My dress had been cut to pieces with a pair of scissors and was covered with glitter paint. My veil is torn to shreds. It's completely ruined.
I broke down crying. I was too upset to even be angry. Alan arrived and found me in a mess. He was beyond furious and went to Rose, confronting her about it. Rose admitted it with a shrug of her shoulders and said "What's the point? She's just a fat old cow anyway." Alan confiscated her cell phone and immediately called Carrie to discuss a punishment.
I don't know how or why, but this is the straw that is breaking the camel's back. I have gone through so much with Rose. I've tried to be caring and compassionate...and this is my reward?
Now that my sadness has faded away, it's been replaced with a boiling anger. I'm tempted to just call off the wedding and leave Alan to deal with his brat of a daughter. This wasn't an accident. This was on purpose. But then I remember that Rose is part of Alan's life.
I love Alan. But I don't know if I can keep taking it.
What do I do now?
TLDR: My future stepdaughter destroyed my wedding dress and veil a week before my wedding and I'm torn between just straight up abandoning him or giving his daughter yet another chance.


Source.