Can my niece sign temporary custody over to me with papers she filled out and had notarized?

She could also revoke such an arrangement while in jail or when released even if she is not in a position to properly care for the child. Better to take the time to do right by the child and set a temporary guardianship in place. Another aspect you have not referenced. Where is the father of the child? And where are his relatives? Even if he has shown little interest to date, the mother being in jail might change that.