Sometimes PCs get rather snobby and entitled attitude, when they have siblings with behavioural issues. Maybe it is too easy to be 'the better or perfect child' and the par is too low for them. Too easy to feel good about yourself when you can always compare yourself to the train wreck. And much too little compassion and understanding that the troubled sibling really does have different cards than they do, that they are not just choosing to be bad while they themselves have chosen to be quick. When good kids make mistakes people tend to compassionate, give them breaks, believe their excuses and giving second chances. At the same time they see, how their troubled siblings are always in trouble, always having consequences etc. Makes them really feel they are truly better human beings than their troubled siblings (or friends or classmates. I was a good kid and I can really remember those thoughts and feelings, it took quite a bit growing up to really understand that those trouble makers in my class for example were not being bad just for the heck of it nor was I any better or more valuable person than they were.) That false superiority takes time to wear off. My easy child has also quite an amount of it going on. Tends to be ugly at times.
It could be useful to let your difficult child know that you are a bit 'on his side.' That you understand these family gatherings are uncomfortable to him and at times his siblings are jerks to him. That you are really happy if he just stays polite and it is okay for him to go to his room, if he feels like it. And that while you would hope he would enjoy himself and have a good time, you are perfectly okay if he wants to socialize only a little. And that you will interfere if his siblings harass him and you would really prefer he wouldn't try to defend himself, because you believe the siblings are more likely to take it seriously, if you tell them that they are out of line.
