The person must have gross TAXABLE income of less than $4,200.
Nontaxable Social security, including disability, doesn't count as income, for the income test, but social security money he spends on himself does count as support not provided by you, for the support test. Money he puts into savings & investment does not count as support he spent on herself. Note that a parent is closely related so there is no requirement that he live with you at any time, during the year.
If no one person (or married couple) provides 50% of the support (for example your siblings are also sending support), then a "multiple support agreement” (IRS Form 2120) can be used, to allow you to claim the dependent. https://www.irs.gov/pub/irs-pdf/f2120.pdf
The IRS has a worksheet that can be used to help with the support calculation. See: http://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf The support value of a home is the fair market rental value, divided by the number of occupants.
