Okay, here is what you need to do. When you are wrestling with her "accidently" grap her tit. Her response from that will let you know right now if you will be tickling her belly button from the inside later tonite when sis is asleep. GO, do it NOW!
alright... i think that my sis is getting jealous of me "stealing" her friend, attention whore that she is, so i think im screwed on chilling w/ her the rest of the night. i'm 15 and shes almost 14. but she is pretty hot and i am kind of desperate. (why else would i be HERE)