To not invite dad’s GF to my wedding?

Fgs, really??

The person who is going to cause all the stress at the wedding is your DM. Sounds like she would share all her woes with your ILs as well which you probably don't want either. It seems to be wrong to exclude your father's gf when she and your dad are not causing the stress directly.

I have sympathy OP, but believe me when you are married yourself a few years, perhaps with a child, you are going to see your father through a vastly different prism.

His infidelity which if I read correctly may have involved close family, a sister? is a truly toxic situation.

As the eldest your mother very wrongly confided in you, perhaps knowing no better.
This adultification of you is undoubtedly very damaging and well done for seeking therapy.

It is ok to say to your mum that you want her to seek therapy as you no longer want to hear about her relationship with your father.
Tell her it is distressing to listen to and it has taken your peace. That you want to be able to enjoy this special time.

Assure her that his girlfriend will not be invited but you will be very upset if either of them embarrass you on the day.
Spell that out clearly.

Your father is really scum and has never cared a whit for anyone but himself, hence his forcing Gloria on you to chat.

You have every right to knock that on the head.

Sadly I see a time may come where the burden they have placed on you results in you not wishing to see either of them because of the stress you associate with them.

The firmer you are with them both now, the better.

But whatever drama your mother is, there is no doubt she has borne a very hard life of hurt and grief at the hands of the scum she married and had children with.

It really is ok to start putting in firm boundaries with them both as you embark on marriage.

Do it before you have a family so you are not dragged into future drama involving christenings, birthday parties etc.

This.
Your Dad precipitated the split because of his infidelity so he has to bear the brunt of this dilemma. He knows having new gf there will be upsetting to your mother and that it will spoil everyone's day.
The gf is not someone you have any sort of relationship with so does not need to be at the wedding. Who knows she may be temporary?
Your mum has been through hell because of his bad behaviour ... her feelings should take precedence. He does not deserve consideration of his.
Just tell him.that you are happy to meet gf on another occasion but your wedding is not the right one.

I think the mum has been very badly hurt and badly treated by the father.
Being cheated on in a ltr or marriage has been linked to PTSD so it's likely the mum's feelings around her exH & his partner aren't entirely rational.

I wouldn't have my adulterous father there, let alone any of his girlfriends, nor blame my mother for letting me know what he used to do. (My father was just as bad as this.)

Oh Gloria was not the OW, I don't really understand why your DM is saying she's stealing her family?

Again, why is it the mum who has to "grow up" and not the cheating dad face a consequence for his shitty behaviour?
And before people say "poor Gloria" for being left out, I wonder if Gloria is that bothered really?

If the person was important to my one parent I would invite them, it is not your fault your parents split and they call act like the adults they are

You don't need to be in the middle anyone acting up on the day would be asked to leave parent or not if I had that at mine

Don’t. I did and she was rude to everyone and my mother threw a huge strop.

I wouldn’t invite anyone that I haven’t met to my wedding.

I wouldn't invite Gloria. Yes it would be nice for your dad to have her there, but not at the expense of your mum's comfortability on your wedding day. I'd speak to your dad and say you'd love to meet Gloria and think it's great that they are turning it into a holiday but you won't be inviting her to the wedding as you don't want to cause any upset to your mum, and obviously, your mum takes priority over Gloria.

It's tricky when it's not really "just a day". Presumably as your family is travelling so far, the celebrations and family activities will run over at least a few days. So your mum and Gloria may come across each other multiple times.

The people you want there are your parents. It sounds like it will be hard enough for your mum just seeing your dad, let alone if "Gloria" is there too.

I think that when someone cheats on their partner, they need to own the impact that has on family relationships beyond just their partner. Your dad should feel grateful that he will be able to be at your wedding on whatever terms you decide and he should understand that it will not have been an easy decision. If he can't accept that, then it sounds like he has no sense of remorse for cheating at all.

If my dad didn’t come to my wedding because I didn’t want to invite his gf of 9 months out of sensitivity to my mum, I wouldn’t think him worthy of being called my dad tbh.

It sounds like your close with both parents.

Can they not all be adults for one day and put their issues aside for you for one day? In whatever that looks like for you. Either not inviting her or inviting her and your mum dealing with it. Have a open conversation with them both (separately) explaining how you feel and take it from there.

also have a long hard think about how the day looks like for you in your perfect world, think about what you want here and go in with that.

Op your mother needs to grow up! My ex was an absolute pig to me for years, cheating, lying, mental abuse etc. His now wife was one of his affair partners and has behaved awfully towards myself and my children for many years, in short i hate the pair of them. However if one of my dc wants to invite their father and his wife when they get married then I would just put up and shut up because I want my dc to happy.

I think it's your wedding and you should invite who you want. If you think having your dad's girlfriend there is going to cause a whole lot of drama then don't invite her. Your dad may decide not to come so are you prepared for that?

But OP is doing the invitee list now. Probably has limited places, and wonders why someone who is currently in a 9 month relationship with her DF should be invited over others she’s probably known for longer, and who she would prefer to have a place at the wedding. As it currently stands, the gf is too new to be that important within the family.

THIS!

OP, are you extremely young???....because a little compassion for your mother wouldn't go amiss.

Your father is cheating scum and has caused your mother endless heartache as she tried to raise a family?

Even shagging someone close to her? A good friend?

Have you really so little idea as to the carnage that can cause?

I wouldn't dream of having your fathers latest piece at your wedding.

I would spell it out to him that he comes alone, or not at all.

I would think it utterly distasteful for your father to turn up with someone.

As for having someone in photos that could be gone a month later, even tackier again.

I think your mother has been through enough with your father, it surely cannot be too much to ask for a little kindness and consideration from her own daughter the day she gets married.

Pretty sure it wouldn't be your father's GF causing the drama, unfortunately.

I would elope if it were me, because I would suspect that my mother, as much as I love her, would make it all about her and her drama and ruin my wedding.

His parents will survive not seeing a shit show.

I'm with you on that. It's ridiculous how unempathetic people are about betrayal trauma.
It takes years to get over and some people never do. That doesn't make them bad people.

If the gf were a decent sort she would bow out of this potential drama anyway.

She's a grown woman dating a man with an adult child he rarely sees, it's not like she's about to become your step mum, OP. I can't see why she would be that bothered about it all, doesn't sound like your Dad is a big part of your life so you likely won't be a big part of hers.
But your mum is your mum and she may well have tried to stick with your Dad to keep the family together.

Are you getting married in a touristy place or is it somewhere quite small? Is everyone from your side likely to end up in the same hotel?
I think how easy it would be for Gloria and your dad to keep clear of your mom generally would influence my plan here.