First you check to make sure she doesn't need her diaper changed, If she hasn't eaten feed her. If she just finished eating, burp her. If all the rest has been done find something from the mom's laundry put it on your shoulder and gently rock the baby.
It could be comfort, unfamiliarity, hunger, illness, sleepiness, irritability, separation anxiety, or the need for interaction. Try to talk to her, play, feed, walk, ,take her outside in her stroller for a stroll, or gently feel around to see if there is some sort of discomfort like a boo boo, or illness. Call the parents if it doesn't work so you don't become too stressed out. Good luck:)
i have a sister this age!(: She might want/need a few things...She needs her diaper changed , she's hungry, shes sleepy, too hot or cold, she wants more/less attention, she wants to be held and if none of these work she might just want to go outside. She might have "special" things to make her stop crying...if so call her parents and ask ;D