Hi @SpinningTops firstly, big hugs, yes it can be tough. Always remember that your DS is probably finding it even tougher and didn't ask to be born with such anxiety and need for routines etc. It's just how his brain works. My DS sounds very similar.
I think, as hard as it is, often with ASC kids we need to radically shift how we think about and view family life, big events, what's important etc. Social media is full of big family gatherings and children behaving in certain expected ways (posing with santa, wearing matching jumpers round the tree) and we're conditioned to do that just as our childhood Christmases were. However, and it's a big however, if one of your children can't cope with that, then is it worth thinking about how you could compromise? Family might not be accepting at first, but tbh, our families were only supportive once they'd seen DS in full meltdown and it's always better to avoid it getting to that stage. So for example, we used to go to my DH's big family get together on Boxing Day. DS was fine when he was tiny as we could put him in his little chair etc. But a few years ago, he just totally melted down: too many adults, too many kids, too much noise, too many expectations. So last year they came to us last year in smaller groups, which was ok, but this year we're going out for a meal instead. We will see!! Honestly, if your DS is ready to leave somewhere, I wouldn't make him stay. Could you or DH take him out while the other one of you both stayed with DD? It's not 'teaching resilience' to make him stay it'll just head into a meltdown as by that point, his behaviour is likely beyond his control. Small steps often work best.
Remember your own self care too.