If your sister is joint on the account with your mother (and she should be) then they each "own" all of the money once it is in the account,
no matter where it came from. Of course, for legal and tax reasons, she should keep accurate records and all receipts...if nothing more than to be able to show to family if there are questions. If there is money in the account and your mother has additional needs, medical or otherwise, there should be no legal reason why that money can't be used. I have never heard of having to account to SS how the money was used. Holy cow, I guess I have to quit using the ATM at the casino!
If you're concerned, help set up a reasonable budget; and give her respite, too.