Just tell it that your sister will come back in a few days ~ ~ and then call your sister on speaker, let it hear your sister's voice to comfort it, and tell it to come back in a few days and not abandon it!

It was winter vacation when my dog first came. At school (boarding), it was crying and refused to eat. This is the way to do it. A few days later, it saw that I really came back, and then it stopped crying when I went to school.