As far as my intuition for English goes, I don't think "peaceful" works here. I think it sounds more like something you'd say about the sea: "Die See ist ruhig / The sea is peaceful"?
I wouldn't use "ruhig" to say that the uncle is "at peace, comfortable", e.g. dozing with closed eyes, or maybe not afraid of dying, if that's what a native speaker would envision for "peaceful".
Or would you use "peaceful" to express that although someone is badly insulting the uncle, he just lets it slide calmly instead of physically attacking that person? But for that, I think I'd rather use "friedlich" in German.
"silent" would work, though. Depending on context, "ruhig" can mean "calm, not agitated" or "silent, not speaking".
I wrote the uncle is relaxed and it marked it as being wrong. Isnt relaxed the same as calm?