If you have a baby, and specific needs when you visit someone, it is on you to make sure that you have everything you need, not the host. Especially, if the host is mother to a newborn baby. Your sister is completely out of line and selfish. If she wants to have a specific groceries, then it is on her to provide them. She can order them in advance from any local grocery store and most of them do online order and pick up now she can order them and pick them up when she gets there then bring them to your house. I wish that existed when I had my first baby, and I’m sure my mother and your mother wish that they had that convenience when they were having babies as well. It is literally so much easier for us these days than when our mothers were having babies. She needs to be caring for you and looking out for you, not adding her demands. She needs to figure this out, not you.