My sister and Mom have both their SSI checks deposited into the same account. However my sister is misusing my Mom’s money. What can I do?

You can call APS and tell them your concerns. They will investigate the financials.

File to become moms representative payee. This will allow you to use moms money for her only. Mom won't be able to touch it and you will have to do an annual accounting for SSA.

Or

Call APS and report financial exploitation of a vulnerable senior.

Many unanswered questions here as to who is POA, whether or not your Mom is competent, and has anyone suggested an Elder Law Visit. This comingling on an account is exceptionally unwise as there will come all sort of questions eventually. I would suggest you see an Elder Law Attorney with your questions.