My goodness @PeeAche, that is awful. Your DH is being ridiculous.
Firstly, your brother in law clearly can't travel to see you all as he has Covid. So he needs to stay where he is because he's self isolating. "I wanted to see my brother at Christmas" is not getting medical care or a Covid test, so he should stay put.
While his parents might not be legally required to self isolate (I think they probably are but things keep changing and I've lost track!), the idea that they want to come to yours when they are highly likely to be infectious would be highly irresponsible even if you weren't pregnant.
As a comparator - my boss vetoed me doing a 30 min home visit to a client who had only had one vaccine last week. That was because of the risk from Omicron - even though there isn't any evidence to suggest the client has been in contact with anyone with Covid. Maybe that was over-cautious, but everyone where I work (NHS) is being very serious about minimising my risk of exposure because I'm pregnant. And I'm a low risk pregnancy too - I might be misrembering, but weren't you hospitalised because of HG a few weeks ago too? That would suggest being even more cautious.
I would tell your DH that there is no way his parents + brother are coming for Christmas when your BiL actually has Covid. His kids won't die just because they don't have the perfect Christmas he has planned. And sometimes shit happens (like BiL getting Covid ...) that means that plans have to change - even plans he was really looking forward to. (As an aside - if his kids get covid and have to isolate, that's going to screw up their Christmas holidays quite badly isn't it? Is that really what he wants?)
If he absolutely insists on seeing them all, I think he should go there and then stay there for his 10 days of self-isolation after exposing himself to someone with Covid. If he won't I'd leave him on his own and go to stay somewhere else.
I really feel for you. My DH doesn't have kids, but he sometimes has a similar enormous blind spot about his family. He's from Egypt and all his family are still there. Most of the biggest arguments we've had have been around me not having the kind of relationship he wants with my in-laws or me having to go to considerable lengths/expense to manage trips to see them and then feeling very anxious because there always seems to be something extra and highly stressful to do when I am already at my limit. It's really hard for him being away from his family so much and to realise that our family might never look like the typical Egyptian family - but that that's ok. Don't know if it's the same with your DH, but I think a lot of the conflict comes from my DH externalising onto me some of his own unresolved anxiety and guilt about his own relationship with his family (and his religion!). He's got better at recognising that and we're both learning to notice when we've taken very polarised positions on something where we both actually have mixed /ambivalent feelings and trying to take a step back rather than arguing. But it's a slow process.
Would it help your DH to see your perspective at all if you really empathised with how disappointed and upset he must be feeling that Covid has spoiled his plans for Christmas with his kids? And pointing out the things that are outside of his control that unfortunately caused this to happen (the new variant, his brother's positive test, his brother already being at his parents so now they all (I think!) have a legal requirement to self isolate). It sounds like he's angry and looking for someone (you) to blame / bully into making a crappy situation feel ok. If he can get to the point where he feels able to say "I really wanted to do X special thing with my kids and I'm so sad and frustrated that we can't" and to feel like you hear that and understand how he feels, perhaps he will find it easier to back down and be able to make alternative plans (that don't involve you, him and his kids all being knowingly exposed to Covid). I know it can feel really tough to swallow all your anger and listen to someone being unreasonable, but it has really helped me and my husband to get on the same page about things where we started really far apart. And he's a lot less unreasonable now!