Cousin is a euphemism for menstruation. Instead of saying "
period",
girls often say "My cousin is here", or "My cousin has come", or sometimes "My cousin is visiting", to express the idea "I'm on my
period".
Amanda:
Girls, I can't hang out with you tonight. My cousin is here.
Sadie: Oh, sorry to hear that. You're having period cramps agaian?
Amanda: Yep. They'are killing me righg now.