My mums a shocker just for letting them eat too much junk food in general - I can't even trust her to make then a sandwich for lunch as she will have given them a packet of chips at noon and she thinks thats enough for them to get by

Maybe you could tell her that they don't really like that brand and offer to buy some to keep there for when you visit, so she isn't wasting her money and make it look like you are thinking of her