How long ago was the one time you saw her? It *might* be fine, if that was a long time ago.

I don't know, maybe a trial run? Will there be a rehearsal dinner? If you go to that and it's fine, then go to the wedding. Obviously if the dinner is a disaster then skip the wedding.

How old are your kids? Are they close with their older half-sister? Do they want to go? Does she want them to be involved with the wedding in any way? Sorry, lots of questions.

I can definitely see how you'd be protective of them when it comes to BM. I never want my DD to be in the same room as BM. I haven't met her yet, but I'm guessing it'll happen at some point. I often wonder what will happen if/when SD gets married. Our situation is such high conflict.

If she acts up ~ walk away be more mature than her.

This day isn't about you ~ it's her day. I doubt she would want to ruin her daughters wedding.

I say go. If it is awkward or hostile, leave. You tried and gave it your best. Odds are good, it has been a long time and she has gotten over it. If it was an affair, maybe not. My parents have been divorced 20 years and my mom still hates my SM. However, they recently have been in the same room for my son's graduation. They kept walking away from each other. Lots of doors in my house, made it easy escape for my dad and my mom. Quite funny actually.

Oh hell no. You go to the wedding, take your kids, these are SD's younger siblings. Be beautiful, be classy, and be prepared for BMs toxic bullshit by scripting your response to various BM toxic scenarios.

Be a family with your DH and kids. That will shut BM TFU, make SD happy, and you will not feed any future drama that may come of your not going.

I do not retreat from the idiots in the toxic blended family opposition. The only way to keep them under control is to keep your happiness in their face constantly.

These idiots are like cockroaches that scurry for dark corners when a light is turned on it a dark roach filled room. You be the light. You and your family be the light. BM will wilt and crawl for her corner.

It may be time to explain a few things to your kids (in an age appropriate manner) about their father's history and his XW. Nothing inflammatory just some information to prep them just in case.

Perhaps some of the anxiety about going is due to your protectiveness toward your own bios. If you do decide to go, then I would advise you have a completely worked out plan of action with your DH as to what if BM comes up to you or them and does anything inappropriate. ie knowing where the exits are, how to round them up and remove them, who can be relied on to step in between her and them (a helpful adult relative?), making sure that person, you and DH are all in constant contact during the event.
I remember the one time that our BM impinged on me physically during the early days of my marriage. She and DH had agreed to have a skid handover outside her boyfriend's home. She had assured DH that she would be at work and the boyfriend would effect the handover. Accordingly, DH said it was safe for me to come too. It benefited us for me to be there as we were all about to drive off for a vacation after the pickup, and the boyfriend's home was on the way to our destination. DH wanted me and him to drive there with our two-year-old in the back of the car and urged me that all would be well. However somehow I knew that BM had set us up and was going to be there, precisely for the purpose of having a confrontation with me. Sure enough, as soon as my DH had knocked on the door, she emerged from the boyfriend's apartment physically holding onto my YSD who was about 10 at the time, screaming at DH, and then made her way over to the car ready to shout through my son's window. Fortunately having foreseen such a scene, I was ready. She, DH and the skids were all out of the car. But I was in the driving seat with the doors locked and my son safe. I quite simply put my foot on the gas and left (temporarily -- rang DH from around the corner and told him to make his way round the corner with the skids when the situation was calmed down). All that my son saw of BM was a face poking through the window and beginning to scream. But before she quite got to the scream, he had already sweetly said, "Hello" and given her a big beaming smile. Was a wonderful moment just before I sailed off to safety. The point of telling you this story is that I had everything lined up to protect us from her, and it worked a treat. Forewarned is forearmed. If you are likely to enjoy this occasion other than the BM being there, I would definitely not give her the power to hex you from going, but I would equally advise you to lay your protective plans thoroughly.

Go. Do not give this woman any control over your life or decisions. Dress your best, be polite and if possible "assign" someone to keep an eye on her and distract her if she starts being disruptive.

Maturity and class win out every time.