If the mother is incarcerated, you know exactly where she is.  If she is "in the wind," she could be dead for all you know.  In the unlikely event she is filing tax returns, IRS will eventually tell you her taxable income. "If a child can’t get the required information about his or her parent's tax return, the child (or the child's legal representative) can request the necessary information from the IRS." (From Pub 529.)  Is the tax result different, if you use zero taxable income for the mother instead of indicating she is dead?  That doesn't make much sense, but then we are talking about taxes.  

Are there any siblings?