Your mother can be a qualifying person for HOH only if you can also claim her as a dependent.

You can claim her as a dependent if she has less than $4050 of taxable income AND you provide more than half her total support.

If she has income from a reverse mortgage, then the support test is where you will need to do some careful analysis.  There is a chart in publication 501 to help.  It doesn't matter how much non-taxable income she has, but it matters what it gets spent on, and whether you or she provides more than half her total living expenses. https://www.irs.gov/pub/irs-pdf/p501.pdf