Maybe he felt she took his parents from him so he would take family from her. Kind of two birds one stone as he was angry with AL. JMO
That's interesting. Given that the victims all directly relate to the suspect through both his sister and Alvin, I think that both connections could have influenced his decision to murder his sister's father in law, her nephew, her husband's stepmother, and his former a former business colleague (perhaps related to a patent).
Did the suspect's sister meet Alvin's son, and then Alvin decided to tap her brother's brain in connection with some patent ideas ... only to learn that he was a mentally unstable convict?