Home mortgage loan and ssi/ssa

Legally, you could, since there is no law stating that you can't.
But that doesn't mean that the bank will accept you as a co-signor. It is voluntary for banks to lend--they can decide when, to whom, under what circumstances, and what they require to issue a loan. The purpose of a co-signor is to make the loan safer for the bank when the primary borrow has bad credit or doesn't make enough money or have a consistent enough earnings history, etc., by giving the bank another person who has to pay--and whom they can sue if the loan is not paid. But if you are on SSI/SSA, your income is "safe" from the bank--if they sued you and you did not pay, they could not garnish or otherwise take your income. SSI/SSA is protected from collections. You therefore are not really someone whom they could collect from, which means that you are not a good co-signor, since you don't necessarily increase the odds of them being paid if the borrower defaults. Therefore, they could decide to not accept you as a co-signor, since it does not really help them, even though the law would let you co-sign.