Can your father get shopping delivered? Maybe take him once a week so he can see what's available, and when you get back you could help him to place an order for his regular things. It might be that you do this once a month, as there is a minimum spend; but once the precedent is set, you have a system in place for the times when you can't take him. 'Ok, Dad, I'm off scuba diving next week, so let's do a two in one order this time.'?
If you accept that shopping twice a week isn't necessary, then maybe that's your sister's point of view too, and with the best will in the world, you can't expect her to do it just because you have done it.
My sister rings our mum on her way to work every day. It suits her to do it, as she walks to work and it passes the time, as well as making sure that mum is safe. I don't work, and suffer from insomnia, so it's not something I would ever have set up - 7.45am is the middle of the night to me. I wouldn't be happy if I were expected to call on a 50/50 basis, although I would happily call later in the day.
I know that this is a different situation; but the principle is the same. You got into the habit of a twice-weekly shopping trip - your sister didn't.
I agree that as a one-off, she seems unreasonable. Thoughtless, in fact. But that is between the two of you to sort out, and will require some very assertive behaviour from you. How you do it is up to you, obviously; but you could start by listing what your dad needs from his children, and share it in a way that suits everyone, then add on the things that you want to do 'just because', and do them without expecting your sister to do the same. She might not realise that you think she is falling short, and a frank conversation could clear the air. Who knows - she might even be feeling pushed out.
Good luck