How can l explain monthly period to a 12year

My husband niece is here for the holiday, her mom is out of the country, her step mum Is always busy and careless. It like am going crazy because I have to explain everything to her.... I don't know if it a good idea to discuss about boys and sex. E.g I spent 3 hours in Walmart today to get her pants, bras and others because I have no idea of her size. Please any tips will help. Am TTC.