You can claim dependents if they meet the requirements for age, custody and support.

For example, you can claim a grandchild if the grandchild lives with you more than half the year, and is under the age of 19, and does not provide more than half their own support, and if the parent(s) don't have a higher priority claim.  (The rules can get tricky and we can provide more details if needed.)

However, you won't receive any tax benefits or refunds for claiming dependents if you don't have taxable income, such as wages, self-employment income, investments, or taxable retirement income.  If your only support is non-taxable SSI, you won't get anything on your tax return by claiming dependents.