SIL who has had Total Knee Replacement

It doesn't sound like that to me.

Or me

Me neither.

I would say "a few days" means about three or four days, not a week - and certainly not a month. A few days is what you were asked to do and that is what you have done. It is totally unreasonable for your sister in law to just to expect you to drop everything to give her more assistance. You have already done a great deal. And, as others have said, many people have these sorts of ops and manage quite well with minimal support.

I think, in the nicest possible way, you need to put her straight.

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.

It is very hard sorting post-op care when you are on your own - I know about this! So I feel sorry for both of you.

I think you should speak to her sister and brother.

I think she is being unreasonable, and she certainly does not need you for a month. I've had both knees replaced, and with the second one I was living on my own yet managed perfectly well. My son and partner stayed just one night with me, and then visited me daily for a few days. Neighbours came and took me out for the necessary walks and took me to physio etc.
It's important to get back on your feet and not behave like a helpless invalid for a month.
She's taking advantage of you.

Update - I rushed round yesterday evening after a day looking after my granddaughter. Her friend had left me a meal to cook for her (nothing for me), so I did that made a drink then went home to get myself something to eat. I stayed the night but said this is the last time. She managed 6 stairs yesterday and with my help 10 today.
I have organised a mobility wheeled tray so can get food and drink from kitchen to lounge. I left her after I did her lunch today with a flask of hot water, teabags and milk so she should be OK until her friend returns this evening.
Her friend is stopping tonight and tomorrow and that’s it but she has agreed to clean for her once a week. I will do her shopping and washing also take her to hospital appointments.
My niece who is very busy has agreed to call in when she can, her neighbour who has a key and works from home has agreed to be an emergency contact. Her brother and sister say they have health issues and are sympathetic but can’t help.
She was quite upset and asked me to just do Thursday Friday and Saturday night but have said no, her staples are out Friday and I have agreed to go round while she has her first shower.
I think this is enough.

It certainly is more than enough.

Well done... She will be fine I had my Hip replacement and New knee and a Pacemaker thrown into the mix all within 6 months for each other, I managed. She's just got to get on with it, you have been amazing.

You are NOT being unreasonable!!
At the very least, she should have told you what this help entailed and just how much she would require you to be there!!
Tell her you didn't know she'd want you back, you have already made arrangements and she should call her other family or friends to help, or pay for a nurse to attend to her!!
DO NOT FEEL GUILTY, THIS IS ON HER!!!

She's probably frightened about being alone , but needs to get 'up and at it'. I had my knees done and my daughter delivered me home, arranged my meds , still with her coat on then left me .......... but it was the best thing she could have done ! I had to do things and managed fine . I'd batch cooked and filled the cupboards prior to surgery so no problems there , showers were taken slower and stairs done with great concentration . I think that it helped me recover faster ?

I had one mid November, went to stay with DD for two weeks post op. The hospital were very pleased that there were stairs in her house and mine. I could shower before I left the hospital. To be honest, I could probably have come home straight away, but my husband is elderly with various health problems, so I went to DD to put his mind at rest. I certainly did not need help with personal care, I think she is taking the proverbial!

I've just been for an assessment and the nurse wanted to ensure I would have someone at home with me after the operation who could cook and generally make sure I was all right.

I had the physiotherapy preop meeting last week. They were adamant that if you can dress yourself, go to the toilet and shower before, then you can afterwards. You will have had a big operation but you are not ill!
They suggested an apron with big pockets to carry food, flasks etc. I could only see a plateful of chicken curry, naan, rice etc going into the pocket. 🤣