Thank you for your advice ladies. OP deleted.
Oh dear! you say you'll be doing a workplace trial, focus on that and try to spend as little time as possilble with this brother of your OH. Also if they have extended family going who don't know the situation, hopefully theyll be forced to behave in front of them...
What's your OHs relationship with the brother like? Could he not have a quiet word with him, even just phrasing it that mum needs help with xmas so lets try to chip in be more positive not put extra stress on her etc? Also maybe he and his brother could have an evening at the pub one night during the 2 weeks, might cheer him up a bit and take some pressure off everyone else, give you an excuse for an early night. Also i would be tempted to 'bring work home' things you need to read for the next day etc so you have a reason to take yourself away if things get to much. Perhaps also agree with your OH that you will have a night at the pub / xmas shopping just the two of you during the two weeks so you have some proper time away from everything. Hope even one of these is helpful as its not really fair for you to not be looking forward to xmas
Just don't go, have Christmas in your own home! They can't make you.
As for the brother, he'd be wise to get another job asap in advance of inevitably getting the sack. (I work for a large company and I've seen this happen before with a comment about gay people).
I would just detach myself from it, I don't really understand why you're getting so emotionally invested in it, it's your OHs family, not yours. I always find when it comes to in laws you have to bite your tongue and rise above them, we all come from different upbringings and we just have to accept how different our in laws are to us. This is coming from experience by the way, there was a point in mine and my H2B relationship where we lived with his brother in their parents house, we clashed and I madey feelings clear, this has now made me the bad guy for all eternity in the in laws eyes and our relationship will never be the same. In hindsight, I should've kept my mouth shut (bit difficult when pregnant with raging hormones!) but I have learnt from this and I keep my opinions/thoughts to myself. Nothing to do with us what the in laws get up to, it's their business and for your OH to handle.
I'm regards to Christmas, you're not obliged to attend. Tell them you're going to spend it with your family and you'll return for New Year.