Okay so the difference is that Sister's with the apostrophe before the s shows ownership. Like that's my sister's dress. It can also be for the fact of my sister's coming over. Meaning my sister IS coming over.
The apostrophe after the s means it's a plural noun. Like it's my sisters' car.
But in the case of this it's most likely the fact of a singular noun being shown with 's and a plural noun being shown with s'. Hope this helps!