Nontaxable Social security 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 himself. Note that a sibling is closely related so there is no requirement that he live with you at any time, during the year. But if you provided a home it helps your support case. The support value of a home is the fair market rental value, divided by the number of occupants. If no one person (or married couple) provides 50% of the support (for example your other 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
