you should ignore him completely and act as if nothing happened (only for the sake of your sister) but then you dont know perhaps he is saying this to other girls too.
i’ll say tell your mom everything and show her all the msgs. dont tell your sister yet cause on the phone you wont be able to explain everything and its likely that she will confront her husband and he will have a good chance to tell a lie or to make her against you.
also block him from your fb account. dont reply him at all.