You start by determining whether you were her employee or whether you were self-employed. How much control did your mother in law have over how you did the work? Did you decide for yourself how and when most tasks you did were done? Or did she mostly direct that?
I think that in this instance it doesn't matter. Such a person reports the income as other income and there is no self employment tax. https://www.irs.gov/businesses/small-businesses-self-employed/family-caregivers-and-self-employment-tax
Okay, so I decided to bear down and just do my taxes so it's outta the way... However I'm still trying to figure out how I report cash payments from my mother in law, I didn't get paid by a company so I don't have an ein number to fill out my tax forms...
The answer was literally posted already.
I assume that you are trying to file using online software? You should be able to report income without the need for an EIN number. If you cannot figure out how to do that, then you might want to consider using a tax professional this year.