Wait... you two aren't married and she can get money from his ss# on your taxes? First of all, if you aren't married there is no way his ss# will appear on your taxes unless he is your dependent under the IRS code.
That would mean he had to earn less than $4000 and/or be related you in one of the following ways.
He is your brother - not likely or she would be your mom too
He is your father - again I hope not likely.
He is your grandfather - I doubt his mother would be alive then.
He's the son of your half-brother
He is your nephew -
He is your brother in law - OK.. that one might work in some way but it is still just creepy.