AIBU to feel uncomfortable if my child invites their father's mistress?

Just send the invite to him only. If he declines then so be it. You’re only civil anyway.

The parents shouldn’t make weddings about them. He should either come alone or not at all. I hope he’s respectful of this.

Did I say it was unique or rare?

I wouldn’t.
Your Mum has had enough shit thrown at her. I wouldn’t want to rub her face in it.
I would tell him she’s not invited. It’s his choice whether to attend or not.

After reading your explanation I vote, yanbu You invite who you want to your wedding. Your results won't read right if people have voted based solely on the original post.
If Dad is trying to blackmail you with his presence then he will lose out.

Hard no and if he can’t put your wedding first for a few hours and attend by himself then I wouldn’t worry about it.
Send him the invite in his name only, it’s up to him then if he wants to come.
Hopefully he’ll come up the ceremony if nothing else.
It would be completely different if he and your Mum had separated, he then met someone and married etc
Instead they chose to sneak around multiple times and cause loads of heartache.
Actions have consequences, why on earth would you want one half of the cause of all that misery there on your special day when you don’t have to.

I get that some posters sucked it up and were brave on the day etc etc but @Zoobaloo mum is clearly not ready to do that especially as this woman has been unkind to her post affair. Some women are vicious to the betrayed partner post affair even though they got their prince. Ultimately, sucking it up and dealing ONLY benefits this man and the woman who has been unkind to @Zoobaloo
mother as @Zoobaloo does not like her or wants a relationship with her. It’s a clear choice.

This.

Loads of marriages have ended with one or the other partner cheating, sometimes whilst the non-cheating partner behaves very unreasonably, or even violently.
The OP isn't in a unique situation, plenty of other people will be in this situation every day.
I've been married a long time, and we are lucky, but let's not pretend cheating is some unique or rare crime.

If is she is actively unpleasant towards your mother, it seems best she doesn't come

You don't like her. She's nasty. You don't want her at your wedding or in your life in the future.
If you invite just your Dad and he refuses, it's job done now and the bad feeling can be dealt with way before your wedding day. If you invite DD and his wife, the drama and upset will go on and on and the worry that she'll spoil the day with her nasty comments will cast a cloud over the whole thing.

Yanbu to feel uncomfortable but yabu to not attend your childs wedding if they're going too. I understand that it's painful being cheated on but show up for your childs big day an don't put them in the awkward position of picking sides, don't be that parent. This issue is between yourself, the ex and their partner/former mistress it's nothing to do with your child. You don't have to speak to them, if in the unlikely event they approach you, walk away and ignore.

My adult daughter gets married next year and myself and DH are in affair recovery.
But if things had gone the other way and he had left me for her, then been invited as a couple next year, it would floor me.
I have to be honest, it would floor me.
I would go, of course I would, because she’s my daughter.
I wouldn’t sway my daughter to make a decision either, as it’s her big day.
But there’s no denying just how hard that would be.

Mummy may not be a saint, but unless its a drip feed she didn't cheat in the marriage. Not all marriages last but as i said I am hardline on marital infidelity.

Sounds like you cant keep both happy so suit yourself, dont invite dad's partner because you don't like her and you aren't that close to dad just civil. If he doesn't come it has less impact on you than if you feel you have disrespected your mum. Also because your dad is the one threatening not to come, I wouldnt give in to that behaviour at the expense of my mum.

The only way dad's partner would be invited is if i was close to her and she is a lovely person. In your situation absolutely not.

Your mother needs to realise that she is there to support you at your wedding. There will be many occasions in the future if you have children where both of your parents will be attending. Your Dm cannot change the past, but she can ensure that the future is positive.
I didn’t see my ex for years and we eventually met up again at DDs wedding, both of us with our new spouses. Yes it was awkward, but I knew how important it was for my DD to have us both there, being civil to each other. Now she has children, there’s many occasions when all parents and spouses are present, and we manage to get along politely.

My advice - speak to your DM and tell her that it’s so important to you that both parents are there, that your DFs wife will be invited and you hope she can understand your feelings.

Ok now I get it, its your wedding and not your kid's. I think in that case its totally up to you. If it was me, and only from what you have told us I'd say to your father that he is not wanted full stop.....but I am hardline on marital cheating.

Good advice for the mother though!

She's not asking him to choose anyone.

She's inviting her father to her wedding.

That's it!

The rest is on the dad and he's being absolutely unreasonable.

I wouldn’t make it about your mum feeling uncomfortable to your dad. I’d simply say, I do not consider your wife family and the way she has behaved is unforgivable so I don’t want her at my wedding. I’d love for you to come, if not no problem.

I am so sorry your father is doing this to you @Zoobaloo.

It is your wedding, and your wishes are paramount. I would have expected a good father to understand why you wouldn't want his mistress there, and why it would cause you stress.

Frankly, is he worth having there if he is going to put his wants above your happiness again?

I agree, except wouldn't make it about upsetting DM. Locate the blame firmly around the new wife's behavioural choices. She's not coming because of the things she's said. Actions have consequences.

Wow.

Well then tell him thats fine, dont come.

Personally, I think he should fall on the sword of discomfort he created. He can do what he likes but he cant expect the world to like it.

It would be an easy decision for me. Dad you are welcome at the wedding but your partner is not as it will upset my Mum and therefore cast a shadow on the day. <fin>

She’s his family. He can invite her to his events. She isn’t your family. You don’t have to invite her to your events.

Maybe not a popular view on Mumsnet but you're choosing to take sides in your parents divorce. Mummy is a saint, Stepmummy is a whore. An unlikely scenario but you'll get plenty of wronged women here who'll agree with you.

If it bothers you and you have no interest in having a future relationship with her then invite your father and not her. He won't come, and if she wrote an aibu saying 'my SD won't allow me to come to the wedding' then she'd be told that he should be on her side.

You've taken your Mother's side and your relationship is over with your Father. You can't control his personal life but you think you can. Move on, and have a life without him. Don't cry when you find his will leaves everything to the second wife.