Help... i live here now...?

so now i live with my "sister" shes my best friend that i grew up with my whole life.. and her father... sunday nite, wen i was sleepin he touched me... n ever since he hasnt said nething bout it he jus acts normal... and he hasnt done it again... im not sure if he will do it again tho... i dnt rlly want to tell my sis but i kno i should... the thing is tho, i dnt kno how to say nething cuz its nvr happened b4... wat do i do?!