How to tell that someone that what he is doing hurts when he is older than you and he is family related.

Well what do you mean by slaps you? Are we talking slaps across the face or other places on the body or love swats to the bum? The thing is if you want him to stop well ask him nicely and if he gets upset well sorry . Or try asking him what he would do if someone was doing something to him that he did not like.