I'm going to tell my roommate his girlfriends little sister and her friend can't be in the apartment
What happened? Why are you banning them from the apartment?
I've never liked strangers inside my apartment.
You can't tell them that! Your roommate also pays to live there so if they want their family members over, that is allowed.
Well then I'll say to him they can't come over anymore.
It is their apartment too, and unless they are being disruptive, then I feel like you are in the wrong by banning them and it could make you ruin you and your roommate's relationship and they may also ban you later down the line from having any friends or family over.