His younger brother (fairly widely known, but certainly not known by everyone) was Peter Graves (of Mission Impossible fame). Peter died just over a year ago of a heart attack.
I was well into my 30s before I knew they were related. Just last week, before James' death, I watched a few older episodes of Gunsmoke. I was struck for the first time how much they actually resembled one another, but especially how much they sounded exactly alike.