Cecilia (my sister) won’t talk to Uncle for a REASON and I don’t wanna get into why with him but he just sucks
Warning: what follows is almost certainly in poor taste.
Options for escalating the situation, YMMV
"I don't know why [sister] won't talk to you [uncle], but you suck so I'm going to side with [sister] on this."
"Keep complaining to me about it [uncle], I can always stop talking to you too."
"You know what you did. Do you know who else knows?"