How to ask a 4 year old if they are getting in appropriately touched at home

Hey ladies! I just needed some tips on what I can ask my nephew about being touched by his father at home. I’ve been noticing for awhile already my nephew likes to completely strip his clothes off all the time which I figured that’s just how some kids are. But every once in awhile he will ask my son to play and they will hide under a pillow fort and my nephew will have his pants off and I’ll catch my mom getting close to touching my nephew. My son is only ever with me or my parents and we always keep a close eye on him but my nephew spends half his time with me and half his time with his father while my sister is at work. His father keeps him inside all day and he is an alcoholic but if my nephew sees my son and my other nephew and niece outside sometimes he will come out of his house without any clothes and rub his bum on the other kids. I just don’t know how to go about asking my nephew if something is going on at home. Do you guys have any pointers?