Wow, so sorry!!! My mom didn't get anything from her mother's house. The older siblings went in and took everything. Amazing what "family" will do under already stressful situations.........but believe me you are not alone or an isolated case.
If your mom had a will made out, you can do something about it. If she had someone as executor of her will then legal action can be taken against your sister if she took things that had been willed to others.
Is it worth going through all that??? Well, I'm a frim believer in seeing that the loved who died
gets what they had wanted at the expense of anybody. But sometimes, for others it is just better to leave things alone. You have a lot of tuff decisions and emotions to deal with......again, so sorry!