I find the best way to save money, is to commit mentally to ONLY buying new things when absolutely necessary. It's simple, and maybe you already do it! But I buy as much as I can secondhand (whether from op shops, on ebay, at markets etc), and I have saved a small fortune by doing that over the years! Also, with stuff for the pantry, I buy in bulk rather than small packets.. also a big saver. Grow some veg too?
See if there is a Freecycle group online in your area - another great way to get stuff you need and save money - and to get rid of stuff you don't need.
Maybe you could swap kids clothes (or yours) etc with other Mums in the area? Just a few suggestions.

The other general suggestion I would make is that if there is nothing tying you that particular region, there may be properties which tick all the boxes for you in a slightly cheaper area. We moved to a fairly remote (and beautiful!) area partly because it was so affordable. Now we work to live instead of living to work

As for your MIL and living in her house... I would be moving out and paying someone else rent until I could save to buy! She sounds like hard work.
and best of luck with all your plans.