My (30f) step-mother (45f) wore white to my wedding on purpose, am I right to cut her out?

This is a bit of a long story, but I'll try to stick to the bare facts. My parents divorced when I was 13, my mother (55f) found out that my father (57m) was cheating on her with my now step-mother (Sarah, not her real name, 45f). My mum was heartbroken by this and so was I. I saw him after their divorce, but only when I was forced to (weekends, some holidays). I was very angry with him the entire time and actually refused to speak to him for about six months.

When I was 15, he married Sarah. I didn't want to go to the wedding but my mum and dad insisted I go. Because of that, and because I was an angry, spiteful and rude teenager, I decided that I'd do something to ruin the wedding. I know, as an adult, that this was a horrible thing to do.

I wore a blue dress to the wedding, but under it I'd hidden a white dress. I went to the wedding with my dad and younger brother (Sam, 25) in my grandfather's car, and when we got to the church, I pretended I had to nip back to the car. I went outside, took the blue dress off, and came back in in a white dress. Both my dad's family and the bride's family were furious, but they obviously couldn't kick a fifteen year old out of a wedding, so they just carried on and got married, but it clearly spoiled it. Even in the photos outside of the church, Sarah looks pissed off.

Sarah's family wanted to kick me out, but my dad held it together and told them to ignore it. This seemed to be going okay until their videographer asked me to say something on camera, and I said, "Let's hope this one lasts."

At that point my dad called a taxi and had it take me home. He stopped insisting I visit him and I didn't see him for a good few years.

My father and I rekindled our relationship when I was in my early 20s. I had a health-scare and we reconnected over that. We've been on good terms since then, although Sarah has never forgiven me, even though I've apologised to her mutliple times (real apologies too. I do feel very bad about what I did to her. It wasn't fair, and I am sorry.).

A month ago, my husband and I got married. I noticed during the ceremony that Sarah was wearing a white bridal-style (not exactly wedding-dress, but similar) dress. When I caught her eye she was smirking at me. I didn't let it affect anything, and just carried on with the wedding. We still had a great time and I think aside from my immediate family, who got the reference, everyone else just thought she was a bit strange.

Later on, when we were reading the guest book, I saw that she'd written in exactly what I said to her fifteen years ago. After both of these incidents, I spoke to my husband and told him that I didn't want her in my life anymore. That I thought, if we had children, that she'd be a toxic influence in their lives, and that I think I'd prefer to cut her out. He agreed if it's what I wanted.

I told my dad that he's always welcome to visit us, but that I'll no longer be having contact with his wife.

My dad basically said "it's both of us, or neither of us" and my mum also thinks I should reconsider for the sake of keeping the peace. Am I overstepping the mark by going no contact?