I’m 16 and doing my cousins wife a favor by taking care of her sister’s kids

Two kids a girl and boy he’s like 1 year old I think and the girl is like 3

Idk what to do

I’ve taken care of my nieces but I feel like kids that you don’t know are bigger responsibility specially cause they don’t know me and idk what to do if they start crying or whatever ... I’ve talked to the little girl before but she doesn’t talk at all.. she literally just stares at you when you ask her something so idk it’s difficult idk what to do