Is the Great Aunt that has legal custody responsible for child support

I am the Great-aunt of a child 15 yrs. old. We live in New York City. I receive Public Assistance from the Department of Human Service (Welfare) for the child. I work and have a self sufficient income so I do not recieve any benefits for myself. I have recieved services for (lets call him Joe) for 8 yrs. I recieved a letter 4/26/12 that his Foodstamps will discontinue because my income must now be budgeted for his eligibility of food stamps. The agency is still willing to pay me for his monthly rent (to which they have done since recieving benefits for Joe) plus a very small cash allowance for the month as well, I am not complaining but the child has a living mother and the agency continuosly have me report to Family Court and Child Support services to update the agency with his mother's personal information such as her where abouts and SS#. Why am I now responsible for his support after 8 yrs. Oh by the way for school and legal purposes the agency made me go to family Court and seek custody. Could this be why. Someone please advise me.