My sister said I should have explained menstruation better to my son

My 12 year old was crying and I asked what's wrong and he said he is menstruating and he doesn't want to be a hermaphrodite.

I told my son he knows girls get their periods. My nephew who is 17 thought it would be funny to tell my 12 year old that if a boys voice doesn't get deep by his 12th birthday it means he's a hermaphrodite and because my son has been more emotional it means he's on his period.

I tried to tell him that isn't true and his cousin lied to him, but he doesn't believe me so I told my sister what my nephew said and told her to plz tell my nephew to tell my son he was joking and she laughed said "Why should? Maybe teach your son about periods so he wouldn't believe something so stupid." When my son was 11 we had the sex talk and I told him about sperm and when a girl gets her period she can get pregnant. The period is the egg and lining and stuff coming out when the egg doesn't get fertilized and an indicator she's not pregnant. How much more clear about what a period is should I have had to be to keep him from believing his cousins stupid lie but my sister thinks this is funny and isn't gonna have him tell him he lied so guess who's making her kid and appointment with the pediatrician so that he can tell him to truth. I guess this will be a funny story in 10 years.

@ariana smols I'm just quoting what he said his cousin said to him.