IS IT NORMAL FOR A 5 YEAR OLD TO KNOW ABOUT SEX AND BABIES?????????????? HELP!?

ok i'm at my grandmas With my 3 cousins. one including my 5 year old cousin. as well as my 13 and 14 year old cousins and i'm 17. (ALL GIRLS). well, my 5 year old cousin has been so perverted. she told me that she likes "wieners and balls". she also told me she think she has per "period" because she was bleeding on her butt. also just a Little bit ago she was laying on the floor and she started humping the floor like a guy would do to a girl if there having sex and she started shaking her butt. the sad thing is she knows shes doing it because she started touching herself and stripping. SHE'S ONLY 5!!!!!!!!! she also told me she likes my butt and i think she has a butt fetish because i was laying on the floor on my stomach and she was laying on my butt reading and she started humping my butt, shell also hump your leg and she knows exactly how a baby is made and where a baby comes from. i don't know what to do or how to confront my uncle! her parents are divorced and separated. any advice. ^_^