It's possible that your sister could get paid for taking care of both your mother and your other sister. It depends on your mother and your other sister having very
low income, and on the amount of your mother's assets other than the value of her home. If your mother's or your other sister's income and assets are low enough, either one or both might qualify for a state-run program where they live, which can provide some money directly to them to pay for care for them at home. They can then use this money to pay your sister who does the caregiving. The amount of these payments isn't high, but they might be doubled if both your mother and your other sister qualify.
These payments work through a state program usually run through a state's Medicaid program. It's called Cash and Counseling or something similar (the names may be different in different states). If your mother and/or your other sister qualify for home care under Medicaid, then Medicaid would normally provide that care through a certified home care agency. But this special program directly pays the person needing care the same amount Medicaid would pay an aide from a home care agency. If your mother or your other sister qualified, they could spend these funds on your sister who takes care of them. They could also use some of the money to make home improvements for their safety or comfort, or to buy personal care items for themselves.
Some states with these programs offer it only to people who qualify for Medicaid under the state's rules. Other states allow people into the program even if they have slightly too much in income or assets to qualify for Medicaid. To learn more about these cash assistance programs -- not all states have them -- go to the page on this site called How to Get Paid for Being Your Parent's Caregiver. You or your sister can help your mother and other sister apply for Cash and Counseling or similar state cash assistance program, if your state has one, at a local Medicaid office. To find the local Medicaid office near you, contact the Eldercare Locator by phone toll-free at 800-677-1116, or online you can go to any search engine and type in "Medicaid" and the name of your state.