This was me 11months ago. Only my DGc are young adults. I set strict boundaries and enforced them. If I had to do it again with you're set of circumstances OP, I'd repeat. My 20yr DGd didn't come near us for 6months. She left home. Wouldn't go to her mum's 2cnd wedding. So sad.
I told my 2 Dd's to keep it away from my door.
I shut down negative dialogue with whoever.
If pre-planned arrangements came into conflict, I left the other one the choice of joining in or cutting their nose off to spite their face. Hard at first,
But got easier through time.
I set the tone and Dh & I navigated a fine line for a few months.
We maintained strict silence between each Dd's affairs/day to day life. Shut dialogue down again quite strictly. Told them both parties as far as we were concerned was not open to be talked about even lightly.
It drove them bonkers. It was like they were toddlers again spitting the dummy etc. Even their OH's eventually adopted this detached indifference. They got no feedback on behaviour. No support, only emotional when they deemed themselves striking towards maturity.
It was a massive brouhaha. Worthy of each other's anger. I asked them to find a way to compromise, grow emotionally and reconcile, or the next time they'd be together in this family home would be for mine or their dad's funeral. Until then they were not welcome. I was as welcome as a fart in a wetsuit, but I secretly cried my tears, put on a strong exterior and now it's getting there. They no longer play their stupid games. We've all changed and adapted. We agreed that the ' normal ' wasn't good, and the new ' normal ' will become comfortable.
Sorry for the tone OP . I hope you find your peace.