They can do either. If the sister just wants to do it herself without her husband completing the I-864a then she needs to make sure she includes proof of her individual income. (I think a tax transcript alone would not show this, I think.)
"This section is used to determine the sponsor's household income. Take your annual individual income from Item Number 5. and enter it on Item Number 6.c . If this amount is greater than 125 percent (or 100 percent if you are on active duty in the U.S. military and sponsoring your spouse or child) of the Federal Poverty Guidelines for your household size from Part 5., Item Number 8. , you do not need to include any household member's income."