Your sister is nuts, ignore her.
If you can only get 12 items you need to get the best value as in the biggest pack of chicken breasts the shop has, a large bag of potatoes, multi packs of canned beans, biggest milk, cheese, carton of 15 eggs etc, biggest available packs of the things you use, but unless you have a good storecupboard, it's not going to last long is it? There's only two of us and we use about 3 dozen eggs a week and loads of bread and milk.
You'd only be judged by your helper if your list was something like
Heinz Beans, if you can't get Heinz in shop one, you need to go to another shop until you find them
Sainsburys brand chicken
Tesco brand bread so you need to go there too
A big bottle of gin for me because I need it
etc
o
Do you have anyone else who can help you (maybe your sister would like to bring round a ration of gruel for you and each of your DC?) or are any of your DC old enough to pick up things from a local shop? If they're allowed out (not sure if the whole household has to isolate?)
Now it’s made me worried about doing the volunteer shop as I don’t want to be judged for my shopping or them thinking I’m buying things that aren’t essential
Don't worry. chicken is a perfectly normal grocery item, it's not caviar FFS. If they think it's not a reasonable shopping item, that's up to them. They are wrong. Ignore them too.