Dude!! I know its really hard talking to them..im 16 and I just dont talk to my sis about much..sometimes, sometimes not. Not because Im shy, embarrassed or afraid to feel stupid, I dont know what it is..
but this isnt about me, I know what you must be going through has got to be hard..talking is hard, just bringing it up like everyone else says..I say your going through something they've all gone through..pick one of your sisters..telling them all isnt a must..the best and most easiest way to talk to them(or one of them)..write a letter and express your feelings that way..its the easiest thing and Im sure she or they would appreciate you wanting to confide in her or them
hope i helped, take care n' dont feel stupid!!