What about things like porridge oats, dried milk, pasta, rice, eggs, flour, butter, loo roll, soap, washing up liquid and soap powder.
Could you perhaps get some things like lentils or dried beans which are cheap but a good and versatile source of protein. If you send flour, your SIL can make her own pancakes and cakes for treats and snacks. She can use dried milk to make porridge and sauces and it lasts longer than fresh.
I'd be tempted to get some mince, some chicken bits, perhaps some sausages and some frozen white fish. Then I would get a couple of bags of frozen mixed veg and perhaps some frozen chopped onions. After that some pasta, rice, tinned toms, tinned tuna, a tin of ham mmaybe some tinned veg like peas and carrots. Tinned fruit like peaches/pineapples/pears. Some cheap teabags or some coffee and something for the children to drink. Flour, eggs, dried milk, porridge oats, sugar, butter, cheese, stock cubes, a bag of lentils, a bag of kidney beans and a bag of chick peas or something, a bag of potatoes and a couple of loaves of bread. Then I would buy some loo roll, soap, toothpaste, washing up liquid and a cheap box of soap powder. If you are careful, you should be able to get all that for around £50 including delivery.
I'm sorry your SIL is in this position, I;ve been there before and it is horrible. I was lucky and my brother helped us out, then. I was overwhelmed and I am sure that your brother and his wife will be, as well. It's a lovely thing to so.