How do you get them to talk?

I went through that with my parents, but it was more a one-sided thing where Dad was trying to use me and my brother to punish our mother. There comes a point where you just have to say, "You know, I love you, but I won't be in the middle anymore. If you have something to say to so-and-so, then say it to so-and-so. It really hurts me when I get stuck in the middle; and if that's they way you want it to be, then you and I are just not going to have a relationship anymore." I'm sorry to say that you cannot force your family members to act like one big, happy family if they cannot get along. Don't play into their traps and let their feelings for each other damage your relationships with them. As far as the family gatherings go, I'm sorry to say that this is just the way some of them are. Ultimately, you really don't want to have all of that negativity around you. If your brother and sis ever WANT to work things out, then they will probably do so on their own terms. If they start mentioning to you about how much they MISS each other, then then (and ONLY then) is there any hope of them ever truly getting along with each other.