I think The Oatmeal has an answer to just about everything in life, including this;
Based on that, this is my understanding:
1. One of my sisters' friends is an actress.
You have multiple sisters, at least one of them possesses several friends, one of those friends is an actress.
2. One of my sister's friends is an actress.
You have a sister who possesses friends, one of those friends is an actress.
The distinction is that in example one you have more than one sister, in example two you only have one.