Am I wrong to be annoyed??

So I live in a duplex. My sister lives in the front house with her two kids and husband. I live in the back house with my three kids. Im a single mom as my husband passed away five months ago.

Anyway things have been tight and then with this whole coronavirus scare going around I was barely able to buy the necessities we need as everything has been out of stock. So the food and things we have are really just the bare minimum. We’re not starving but we’re not feasting either.

My niece and nephew have a tendency to come over whenever they want. One is 5 and the other is 6. They’ll bang on my door until I let them in and obviously im not going to ignore them if im home. But as soon as they come in theyll make a mess despite me telling them to be careful with the toys and put whatever they use back. They dont listen. They’ll take the toys my kids are playing with and as mine are younger they basically get bullied. And if i try to interfere nicely they wont listen. Im also not in the business of disciplining other people’s kids, and i know it would cause a problem even if I tried.

But has been getting to me lately is they ALWAYS raid my kitchen! They’ll come and ask for baby teethers, yogurt melts, cereal, and basically anything they see. I dont deny kids food so I’ll say “here you can have one but if you want more than a snack go tell your mom” Do they listen? No. They’ll grab whatever they want and run home with it. And do their parents say anything? No. What annoys me more is if their mom (my sister) comes to check on them, they’ll take her to the kitchen to ask for food. Instead of saying they have food at home or even asking, she’ll go ahead and help herself and give them whatever they want. Without checking ! Mind you, I don’t even go across the yard to their house uninvited or without calling to ask. Let alone just waltz in and help myself to whatever is in their kitchen. And neither do my kids.

Both her and her husband work very steady jobs making easily twice more than i do, in fact they own the duplex we live in. I don’t go over often but when I do their kitchen is always overflowing with food. In fact yesterday I was watching their kids at their house while they went to the store and they came back with a bunch of groceries as well as pizza. My four year old asked for a slice and they ignored her, so i told her it was okay we would go cook at home. I don’t expect them to feed my kids but if the roles were reversed hers wouldn’t even ask. They’d just take and she would let them. It’s little things like that that annoy me.

I want to say something but dont want to seem like a bitch. Am i wrong to be annoyed? Advice please