Before she was discharged from hospital, she would've been asked if there was someone at home to help her initially. Usually, for someone living alone, with no one to help, the OT can arrange for some short-term care ... probably only for the first few days in a case like this, until they were confident she was capable to continue on her own. They wouldn't be standing for any nonsense, and would encourage her to be doing things for herself. She obviously told the OT at the hospital that she had someone to help her out for a few days ... YOU!
As others have said, don't stand for anymore nonsense. She needs to be encouraged to things for herself, otherwise, if she sits back letting someone else do all the work, she'll never regain a normal level of mobility. She needs to keep moving!
You absolutely don't need to be doing all things she's expecting of you! My dad had 5 knee replacements (yes, 5, some had to be re-done due to complications!), and he still made every effort to do everything for himself.
At the very most, it should be OK for her brother/sister/niece to take a turn to just pop in once a day to check on her, and to see if she's anything. After the first week, she could simply call someone if she needs something, or just do her shopping online.