I've got family that always seems to be getting into a tiff here and there and I'm always stuck in the middle of it like the mediator. Something about the holidays always seems to make it worse. Talk to the brothers and see if one of them is willing to extend the olive branch and make peace to the other. If someone does that than maybe there is time to find a way to be nice to each other. If neither is willing to stop being mad than there isn't much you can do but let it blow over. It does stink that the kids might not get to see each other and it's a shame that the parents can't see what it does to their kids to have feuds like this. Unfortunately adults can act worse than children sometimes.
Like I said, talk to each brother and find out if someone is willing to bend. Use the kids as a way to tug the heart strings. Remind them that it's the holidays and even if they have to just "act" nice doing it for the kids would be the right thing to do. But don't push too hard. If you were an outsider you could just say your thoughts and walk away but your in the family too and you don't want to get drawn into their fight. Make sure you stay neutral and let them know before you even start talking to them that that is where you are going to stay. And if it seems like what you're doing isn't going to work just let it go. Families fight. VERY loudly sometimes. Staying neutral will help everyone. If they ask your opinion give it if it's not going to make the situation worse. But don't be afraid to speak your mind about the holidays and the kids seeing each other. Thats an obvious reminder that both brothers seem to need. If they tell you point blank to stay out of it then do that.
Lastly, I would clear it with your hubby first before you talk to his brothers. Just let him know what you're doing so he doesn't get ambushed with phone calls later or something. He might have some insight on how to deal with each brother separately or he might just want to do the mediating himself.
If it all doesn't work out try to have a good holiday without them and know that you're doing the right thing by trying to help, even if that means staying neutral.
Good luck! Love to hear how all this turns out.
Jen
http://www.mommysjoy.com
OH, and one other helpful thought. When I'm playing mediator (it's my two sisters that get into it all the time) a lot of the time I will act like they are the first person I called. What I mean is, the person on the other end of the phone will want to tell you their story without thinking that you've already been tainted with the other's story. Sometimes it's just better to let them think they are getting the first word in. It doesn't always help in every situation but sometimes they just need someone to vent to and if you aren't "tainted" then they can feel like they are talking to a non-biased person.