Well, I think I did a good job. My sister told me they were asking if Uncle X was coming this weekend, and so I figured it'd be good for me to tell them.
We were all swinging on the hammock, so I asked them if they had friends in school whose parents don't live together anymore. They said yes. I then said, so you know what divorce is? Yes... OK, well Uncle X and I are unfortunately getting a divorce so you won't be seeing much of him anymore. The older looked at me - are you kidding? Nope, sorry. Then not too much after that. I'm sure it's sinking in and there will be questions to my sister. But then again, maybe not. Kids can be surprising.