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Thanks for the additional information. Since you all live together, you or your sister could claim your son as a dependent, as long as he didn't provide over half of his own support. However, the IRS has a tie-breaker rule when the dependent could qualify for more than one person:

Tie Breaker Rules

Sometimes a child meets the rules to be a qualifying child of more than one person. The following rules must be applied to determine who can claim the child as a qualifying child. Under the tie-breaker rule, the child is treated as a qualifying child:

• The parent, if only one of the persons is the child's parent,
• The parent with whom the child lived the longest during the tax year, if two of the persons are the child's parent and they do not file a joint return together,
• The parent with the highest AGI if the child lived with each parent for the same amount of time during the tax year, and they do not file a joint return together,
• A non-parent, if no parent claims the child as a qualifying child although he or she may do so and only if the non-parent's AGI is higher than the highest AGI of any parent who may claim the child, or
• The person with the highest AGI, if none of the persons is the child's parent.

This information is found in Publication 501, Dependents, Standard Deduction, and Filing Information.

According to this IRS worksheet (which is an extract from Publication 4012), If a child receives Social Security (earned from working) benefits and uses them toward his or her own support, those benefits are considered as provided by the child. Benefits provided by the state to a needy person (welfare, food stamps, housing, SSI) are generally considered support provided by the state. So, if his payments are SSI, then he didn't provide his own support.

So, your sister could claim your son as a dependent if her Adjusted Gross Income is higher than your and you agree not to claim him. Otherwise, you would claim him as your dependent.