AIBU to not invite my 20 year old son on a trip with his siblings?

This!!
As a person whose parents divorced when i was mid teens due to my mum having an affair and leaving my dad, I did choose to live with my dad and ignored my mum. HOWEVER (and the huge difference here) he actively encouraged me to ahve a relationship with my mum, facilitated it and had a go at me for ignoring her. Basically, he behaved exactly as a grown up should behave.
You have not.

My mum was like this when she got divorced (although she has the affair), she expected me to move with her and I stayed with my dad. Not for particular loyalty but I was 17 and didn’t want to change my life. She called me all sorts and accused me of betraying her for money.

Your son was clearly struggling and you decided to ignore him which seems absolutely wild. You can disagree with him but have a conversation instead of you and the siblings all ignoring him.

I am completely no contact with my mother and she has no relationship children.

I never wanted my kids to pick sides. But I won't lie, it stung. I expected a bit of loyalty, not because he owes me anything as his mother, but because his dad cheated on me and lied to my face for who knows how long

But he's also an adult who chose to stay with the parent who broke this family

The son said some things that were wrong and if a reconciliation does take place he should apologise for them.

But it's clear from the quotes above that his real offence was to stay with his father and the claim that the OP didn't want the kids to pick sides is obviously nonsense.

He reached out - perhaps to apologise and mend your relationship - and you gave him the cold shoulder as a punishment.

You wanted him to choose sides & he failed the test.

It is hardly surprising that some men learn contempt for women when they are permitted to treat us however they please.

Correct your sons before they presume it is their right to diminish, damage, or destroy another.

There’s also a difference between not indulging someone, and cutting off your own child.

A couple of thoughtless comments from someone desperately trying and preserve the family they’re seeing fall apart, do not constitute a malicious attempt to manipulate. OP wasn’t the only one raw and in pain due to the end of her marriage.

I’m surprised a lot of people are choosing to gang up on your for being hurt/going cold while conveniently ignoring the fact he has no issues with his father’s behaviour BUT he’s already warning you you’re not entitled to a life of your own in the future.

He sounds very manosphere/red pilly and needs a reality check.

I think it already is. If I were her son I'd find it very hard to come back from this.

Some of these attitudes are why men grow up to be cheaters in the first place. Babying a 20 year old for being unfair to his mum over his father's actions is not the right course of action.

You clearly DID want your kids to pick sides, your behaviour towards your eldest makes that crystal clear!! 😂 I’m not condoning his behaviour since the split but you need to take responsibility for your significant contribution to the state of your relationship with him.

yabu. You are the parent here. He is very young still. You need to start including him and inviting him to everything-days out, holidays, family therapy etc
otherwise your relationship will be irreparably damaged

Look at the dynamics in this family. Father who cheats with a colleague and lies to his wife's face. Mother who expects children to pick sides and punishes one who doesn't choose to follow her by ignoring his efforts to reach out and rewards the children who gang up against him by taking them on holiday and posting photos for the outcast child to see. That's some stately homes territory in the making.

There is a fine line between allowing grace and being emotionally manipulated, which OP's son is doing.

Shock. Horror. The OP does not want to indulge his behaviour and protect her other children.

What a heinous crime!

Yet OP is now complaining and wailing that she doesn't know what to do. SHE is facing the consequences of ignoring her son's attempts at contact and then excluding him from a family holiday.

I have two sons and the difference in emotional maturity from 20-25 is really stark.

I think he probably needs the OP's input and parenting more than ever, considering his views and his Twat fathers behaviour.

Women/mothers being held to a higher standard than men/Dads, is surprisingly common IMO.

We can push back hard though and not accept it.

It's terribly worrying, too.

To think that women must be martyrs and endure tribulations.

Giving grace to your clearly hurting child, you mean.

He’s 20, yes, and with little life experience. He didn’t know how to perfectly navigate the destruction of the family unit he’s always known, and said the wrong things whilst trying to.

The point you have made about how devastated there were at the time but how much they have come to realise that it's always been mum who is there for them is really relevant. That is what should happen. You let the children have a relationship with the other parent, don't take sides and they will see it for what it is as they grow older. Your talking about 20yrs ago, OP has barely been separated for 6mnths.

I was reading this:
I feel like I have no family and everyone hates me, I'm the outsider.
and I thought, well, mate, you made your choice.

But then I went back and reread your post, and saw this:
My eldest tried reaching out a few times and I'll admit I went quiet on him for a bit. I wasn't ignoring him out of hate, I was just raw.
So did he stop reaching out? Did you reach out to him? Have you reached to him since?

It's only been five months. He is 20, his brain is not formally formed, he will be impulsive, and he may have had many reasons to stay with his dad: nearer his gf/college/work, maybe just him and his dad and not a busy house of six, who knows?

Have you left the door open for him to be with you, or just cut him off? People are allowed to change their minds.

I think your daughter was unfair to just invite her dad, but if you don't do the grown-up thing and reach out to him, a coffee, a conversation, listening to his side, maybe putting your side without rancour, then there will be a rift that will get harder to mend the longer it goes on.

I firmly agree with you and commend your friend's strong resolve.

The OP is shielding her other children from her son's toxicity and vile behaviour. I would not want him around.

He shouldn't have to pick sides. He should be allowed to love both his parents.

I think you're both a fault.

He has no right to dictate how you live your life. Equally you've been ignoring him and that's hurt him.
You say he's grown up so are your other son and daughter.

You need to be the parent and have a chat with him.

I think we will increasingly see this as young men are sucked into the manosphere.

A 20-year-old male who clearly aligned himself alongside his adulterous father.

He had already chosen which side he wanted to be on.

You are not wrong in your view.
I think the OP has every right to firmly defend herself.
Her husband is a cheating twat whom has cheated on his wife AND his children.

The OP has every right to spell this out to her children AND to make it clear that she will not accept blame for implementing consequences for their fathers cheating.

It is surprisingly common.

When this happened 20 years ago to a friend I play tennis with, her two daughters, mid teens at the time, were devastated at their darling father having to leave the family home.
They blamed her for not being able to forgive Daddy his year long affair.

They are now both married with young families themselves, and their views on both parents have long since matured.

They are still fond of Dad but respect wouldn't be the first adjective they'd use to describe him.
They absolutely adore their mother and her strength.
She never badmouthed him despite being deeply hurt, but she wouldn't entertain forgiving him and trying again, when his affair with his younger colleague eventually ended.