It might be best to self report EVERYTHING to CPS.
You indicate that the child is not related to you, other by a marriage.
A step-ANYTHING is a LEGAL nothing.
Therefore, that fact alone is reason enough to IMMEDIATELY report the complex situation that a relative abandoned the child to your custody, and has since disappeared.
Children aren't puppies, used automobiles, or bicycles; to be passed around willy nilly.
Contact CPS to help the child and yourself, because you don't know what's really going down behind the scenes.
CPS can assist you with obtaining some form of TEMPORARY / EMERGENCY CUSTODY so that you can eventually be granted a more permanent CUSTODY when CPS works things out.
If you don't contact CPS, an angry person, let's say the SPERM DONOR daddy might say you kidnapped HIS child.
Or, you can HIRE and PAY for an attorney (out of your pocket) to seek some form of emergency custody.
Good luck.