Welcome to the forum!
I'm sorry to hear you lost your dad to
ALS (or Kennedy's Disease). If, as you say, his brother and first cousin are showing signs of having
ALS, it is something they will not be able to ignore for too much longer.
I do not see what harm you could do by statiing that you notice they seem to be moving with more difficulty. Who knows, maybe they both need someone to confirm what they may be suspecting anyway. I'd bring it up but that's just me. Nowadays, when I see someone with a cane, in a wheelchair or just walking funny, I'll ask them about it. So far, no one has socked me in the head for asking. I'm waiting for someone to tell me they have PLS, I guess.
Zaphoon