Congratulations! It's so nice of you to be thinking how you can help too :)
Chat to your sister to agree days/ times etc and stress to her that you are coming to help so that she can concentrate on DC and herself. As long as she's happy then crack on ! :)
It will be wonderful for her to know that she doesn't need to worry about the washing/ hoovering/ housework/ cooking etc, whatever she needs. I'm sure she will have many other visitors who'd want to call in, but she maybe doesn't want them yet as she thinks the house is a mess and she can't get a shower in peace etc etc
Perhaps you can even take her washing etc away and return it clean and ironed ? i.e just a flying visit to drop off food and collect washing if she's not up for anything else ? You don't need to stay at her house to help with some of the things if she wants peace and quiet.
Absolutely a giant YES to the household tasks, freezer meals, little luxuries like nice shampoos and chocolates.
The thing I found hardest was changing my DC's nappy through the day as the changing table was upstairs in the nursery. My gran gave me a large pouffe which was on wheels and I could have it in front of me, put a changing mat on it and still sit on the sofa while I used it without having to twist and bend. Also , having a changing box at the side of the sofa is super handy.
Snack pots with fruit and nibbles are also very handy if breastfeeding, cos you don't get to move much while feeding and you can get really hungry.
xx