We don't have anything that serious in the immediate family, but I have an uncle who is not speaking to any of his siblings but is on good terms with me. I've explained to my 9-year-old that Uncle got so mad at Grandma that he just didn't want to see or talk to her anymore; it's very sad because Grandma is so nice and Uncle is really missing out, but this is his decision and we need to respect it.
We also had a friend, who had been one of my partner's best friends since elementary school, who got mad at my partner when our son was 4, had lengthy arguments with my partner and me while mostly ignoring our son (with whom he previously had an uncle-like role), and finally after a year of this came to see us and acted ready to make up and told our son he'd see him soon--but then changed his mind and cut us off. This still comes up every few months, 4 years later, whenever something reminds our son of this friend: "Why won't he see ME? Why doesn't he send me birthday cards anymore? He never had a fight with ME!" I tell him the guy just got so mad he wasn't thinking straight, and now, we don't really know what's going on with him, but maybe he's embarrassed by how he acted. Also, I've said that although there are many things we liked about him, he really really hurt our feelings and that's hard to forgive, and I bet he feels the same way about us. Sometimes it's better not to hang around with someone who keeps hurting you.
My other uncle (not the one cut off from his siblings) was married for many years to a woman who gradually became an alcoholic and tranquilizer addict and dangerously neglectful mother. I would have appreciated more explanation of what was going on there, when I was a child. What I overheard from adult conversation about her was perplexing, and some of the things I saw her do worried me. When I became an adult, my parents and other relatives began disparaging my aunt when talking to me, but they still didn't really explain, so it was not until I was about 25 and her son came to visit me that I realized just how horrible their nuclear family's situation had been for years. I wish I had known. My son has never met my now-ex-aunt, but I've done my best to explain addiction
to him so that it won't be such a foreign and shocking concept as it was to me. I've also told him about my special visits with my grandparents when I was a kid and how they were sometimes marred by my bratty younger cousins coming to stay for several days, and how I understand that better now that I know they were there because their mom couldn't take care of them and their fear about that was probably one of the reasons they acted the way they did.