I wonder if living with the in laws for so long has skewed things a bit for them.
It's. Bit worrying they weren't able to save in this period but I'd assume when your granddaughter was small your daughter worked less.
But when you don't have your own home and little control over these areas it's easy to develop a bit of a Next habit because DGD needs to look nice/needs new clothes (and it gets you out of the house).
You don't have to do anything wild to start eroding income. Buying new children's clothes from these places is expensive. Mine wore mostly second hand or things picked up cheap in supermarkets etc.
But when you feel like you can't provide your own home, it's common to compensate in other ways. Studies have shown those on the lowest incomes the most resistant to charity shops for example because it's tied up with status.
I wonder if they've got into a few of these habits and now see it as a need. If so, you need to introduce her to Vinted.
Couple this with a couple of expensive car loans (are they PCP or loans to pay them off? Hopefully the latter) and money can run through your fingers.
If her in laws are better off they may also have GD likes heinz beans or kellogs cereal type habits that force up the shopping bill. Chinese on a Friday is a tradition in their house and they've carried on etc.
They probably have quite high insurance costs on their cars too but they are getting older so hopefully have shopped around.
I don't think it will turn out to be anything crazy that they are doing, just a small overspend in lots of different areas and a slightly skewed version of essential.
It's probably a good thing you've got to this position as a bit of guidance might help them considerably in the longer term.
The money saving expert boards are excellent for a statement of affairs/budget to see where costs are high.
They also provide good budgeting advice because it isn't just about what goes out every month. You need to factor in annual costs like Christmas, birthdays, car repairs and divide it all by 12. I think you said one of them is paid weekly which can also make budgeting harder if you are not on top of these expenses.