For Mark Sheridan, an attorney whose father and mother were killed in their Somerset County home in a homicide that remains unresolved after nearly a decade, Monday’s indictment of the political boss George Norcross tore open old wounds. One section of the indictment in particular — the one involving his dad. It describes a real estate deal that created a serious rift between Norcross and John Sheridan, Mark’s father, a deeply respected advisor to governors in both parties. Norcross and the senior Sheridan had been friends, but in the months before Sheridan’s violent death, he told his son th…