Unless you are the beneficiary, they won't tell you. If you find the name of the insurance company, you can call and ask if you are the beneficiary, and your sister can call and do the same. Best bet is to dig through her papers to try to find the policy.
If you know which insurance company the policies may have been issued from, you can contact the insurer directly. If not, there are some services that can help you - visit my blog and search the term "policy" and you will find an article with links to additional help (I can't post links here). Good luck, and very sorry about your mother's passing.