How do stop my younger sisters from...?

she's playing the habbo game *something like zwinky (an avatar game)* and she is only 8!!!!!! i'm 14 and i don't think it is save.. even for me... my other younger sister (she's 12) and she actually encourage her to play the game... how do i stop them?! they are SUPER DUPER HYPER stubborn and just won't listen to me.. and everytime i tell them the internet is not good for KIDS LIKE US, she's all >> what's wrong with making new friends... bla bla bla..