Alcohol at my party?no parents.. spring break??

im havin a party over spring break my parents r going somewhere but my gma is coming on sunday or i was thinkin have it on friday so ill have enough time to clean it up and stuff... but my friends want to bring alcohol to it- i dont drink but my sisters will be there- up in there room but there 7 and 9 and idk if they should bring it cuz they will be here but i cant just say no- cuz al it will already be here- so how do i control people if they get drunk or whatever when my sisters r upstairs so my parents or gma wont find out?