Your sister should come to NY and file a Guardianship petition, naming both you and the other parent as respondents. If both parents agree, the court will take her fingerprints and run a background check. A lawyer will be assigned to the child. If he is over 14, the court will require that he sign a consent called an Order of Preference. Assuming all is clear with the background check, and the other parent also consents, a guardianship order can be issued on the next court date.
You would have to check with the school district in MD to see what they would need to allow him to attend school there. You also have to look at whether there is a non-custodial parent and whether or not they agree to the situation. Good luck.
I have been a criminal attorney in New York for almost 25 years. website: Brooklynlaw.net Phone #: 718-208-6094 email: firstname.lastname@example.org. This answer is only for informational purposes and is not meant as legal advice.