Adult children do not get along - how do you cope?

I think you have to 100% step back from any hint of taking sides. It only makes it worse.
You will never be successful if you try to correct them in their interactions with each other or mediate. You are too close to it and they will always be trying to ‘win’.
I would have lots of stock phrases for when you listen to them vent. ‘That sounds tough’. ‘You both sound like you have some good points’ ‘I am sure / I hope you guys can work together to sort this out’
Tell them you have recognised that they are both adults now and need to figure this out for themselves. Tell them ahead of time that you won’t try to fix it for them.

Then physically leave the room when they start in on each other. Or ask them to leave the family space if they are arguing.

Refuse to be pushed into you being the parent and them back being your kids.
They are adults and need to work it out for themselves.

Unless you really think that one of them is the main aggressor/ a bully to an unacceptable level and then maybe that one needs to leave home again??

Assuming that one of them isn’t the main issue or aggressor, you need to stay out of it. I’ve never liked my brother. My mum interfering to try and make us like each other would do nothing to help. They don't need therapy just because they don’t like each other and don’t like living together.

Be completely neutral and focus on having good relationships with each of them separately. Refuse to get involved in disagreements and explicitly tell them that you won’t. Limit how much time you make them spend together. Unfortunately, you don’t have a happy family that will spend happy family time together. But you can have two separate happy parent-child relationships independent of each other.

Reminding them that you’ll die one day and “they’ll only have each other” is pointless. And not true- if they dislike each other that much, they won’t “have each other”, they’ll likely just never see each other again.

I am an adult sibling who doesn't talk to my DB unless I have to. Also both ND. And when my parents die I probably won't talk to him again. My DM took sides, mostly his. And made it immeasurably worse by pushing. She wanted to make me feel guilty and ashamed. Which made me avoidant.

Take a huge step back. Insist on manners and consideration just like any shared space. And see what happens when they move out.

I think this is far more common than people realise. You need to stay completely out of it and stop trying to mediate between them. They are adults and can decide who they want in their lives, close family included. I would also say you don’t really know exactly what’s gone on between them. My dh is completely non contact with all of his siblings - all 3 of them including his twin sister, for various justifiable reasons. You can be a mother to both of your dc but stop expecting them to have a relationship between themselves if they don’t want one.

As awful as it is, you need to step back and accept it. It is what it is and you can’t mend it.

My DH is one of four siblings and two of them no longer speak to the other siblings (or each other). I feel sorry for MIL, but her insistence of therapy and always trying to orchestrate the situation has been tiresome and useless.

One does not go to any family events or to MIL’s house when other siblings will be there, they just see MIL on the side. The other also doesn’t go to events but will tolerate being in same house as other siblings, but conversation is awkward, stilted and ends as soon as necessary.

They won’t ’only have other’. They will have partners, friends and their own lives.

You said that the issues related to how you treat them as teens - therefore I think it is maybe within your own remit to repair the situation

Have you apologised

One thing: DO NOT harp on and on ant them about how you wish that they were closer! My mother tried this, until I said bluntly that I hadn't liked DB since primary school, we live 100 miles apart so are unlikely to see each other, and quite frankly, she and her brother weren't close, so she wasn't exactly the best example!

I kept out of his way when he was around, often in my room.

We got on better when he moved out, and by 'better', I mean that we could have polite, civil conversations- the kind that you would have with a stranger.

We don't speak (he is glued to his phone, but somehow manages to never reply to any messages, so SIL plugs that gap when it comes to my niece and nephew) and only ever see each other at family occasions. We're two very different people, in looks and personality, and if we weren't related, I don't think we'd even care to know each other.

What exactly is the practical problem here?

Do they actively argue, make life at home unpleasant, refuse to be in the same room at same time? Or are reasonably civil day to day but are not interested in being friends/close? Those are pretty different scenarios requiring different reactions.

It’s pretty common for siblings to not get on as adults, and it’s not really something you can control or force.

If you remove the pressure, then in future they may be able to get on ok in short bursts - I have one sibling that I’ll happily see for a few hours if our parents host Christmas for example, I’ve got no issue being civil and even amicable in that context. I just don’t choose to spend any time with them the rest of the year!

But I have another sibling who I do not like at all, and there has been huge family pressure for me to look after them (they do need some degree of care, I’m not able/willing to provide it). The more they push, the more I pull away from them. That situation has actively damaged my relationship to both parents, I’m currently barely speaking to them which is very sad but I can’t handle another conversation about sibling’s needs and what they think I should be doing.

If you step back, treat them like two adults who have to cohabit politely and considerately but otherwise don’t owe each other anything, then gradually they may be able to repair their relationship.

How do you know that they both love and miss each other? If they've both told you that then maybe one day they'll grow up and be closer but if they haven't then maybe that's just the way it'll always be.

I always thought I was quite close to my sister but a few years ago she slammed the phone down and we haven't spoken since. Her behaviour since has made me think she's not a nice person and I don't even like her let alone love her. So far we haven't had any big occasions where we have to see each other. She never visits my mum so no chance of bumping into her there and I get together with her grown up children separately.

But I have sympathy with you having to live with it. I think providing they're civil to each other there's nothing you can do, if there's constant tedious unpleasantness I'd be telling them to make an effort to be civil or find somewhere else to live.