This may seem like an obvious answer (it was for me), but have you considered starting your own daycare business from home? I did that for a while when I was in school... stayed home with my kids, watched other kids (playmates! so much fun for my babies!), and had a part time job just so I would file my taxes at the end of the year. It was exhausting some days, but I made it a point to help out single mom's who couldn't afford 'real' daycare, and emotionally and financially, it really paid off in the long run.
Also, I was fortunate enough (when the economy was good) to work from home for a trucking company. THAT was a LOT of work (as I was the only employee other than the drivers and the owner)... but man, it was fun. I worked 7 days a week, and long hours, but I was HOME... trucking has gone down since no one is buying, but it's always worth a shot. Look online for over the road trucking companies in your area that are looking for dispatch people... most places are small ma and pa family companies that don't mind their employees working from home, since then they won't have to spend the money to rent office space.
One other thought, only because this is what I'm doing now, look up online any websites in your area that may need help answering support emails. I happened to get this particular job because my kids godfather owns the company, but there are tons of websites that could probably use the help, and since it's all online, no phone support, you can do it from home, in your jammies, while doing laundry, with kids screaming in the background. The pay isn't glamorous, but it's a paycheck, and when you factor in the money you're not spending on daycare, driving too and from work, work clothes, lunches, etc... that smaller paycheck is totally worth it since you're not spending most of it on those things!
I know I didn't touch on the MIL thing, but really... sounds like she needs to get a grip and realize everything you are doing for her son and your family!