Two siblings were arrested on charges of murdering a Newark grandmother, who was gunned down on her front porch as she celebrated her birthday last year in the state’s largest city, authorities said Monday. Jonathan Ritchie, 21, and his brother Josiah Ritchie, 19, both Newark residents, face charges of murder, conspiracy to commit murder and weapons-related offenses in the March 11, 2021 slaying of 63-year-old Debra Derrick, according to the Essex County Prosecutor’s Office. “The two shooters were identified as brothers living with their mother and stepfather,” Acting Essex County Prosecutor T…