How Can Inappropriate Behaviour In A Child Be Controlled?

Hi I am the mother of 4 children the youngest is my only boy and he is eight. There is a significant age difference between him and his 25 year old sister and 20 year old twin sisters. recently the 25 year old has come back home and on two separate occasions he has flashed her with his penis. he has called her attention and flashed her. I addressed this behavior as inappropriate behavior because one his sister is uncomfortable two its his body and it should be private. Im concerned if this is normal behavior. he does it while laughing because he knows he is going to get a reaction and thats his personality anything for a reaction but this is different to me and want to know if I need to be concerned.