Usama's twin brother Sami is a member of the Executive Force as well as Hamas.
Losing his twin brother and friend has shocked him to the core. He is torn between a brother lost and political affiliations held dear.
Sami said: "We used to argue together about him being from Fatah and me being from Hamas. It was normal. He would joke around and I would always tease him."
Brothers in conflict
The entire family is still coping with the loss, which brought home the magnitude of intra-family political differences.
Adnan, Usama's father, said: "We're all brothers. I just never imagined that Usama would be killed at the hands of a fellow brother. For what? What had he done?"
Um Raed, Usama's mother, said she had surrendered her loss and pain to God. Now she must battle her own feelings of anger towards those she sees as her son's killers.
She said: "When I got the news, I was struck. I kept praying that he was just slightly injured or that it wasn't even Usama. I asked God to give me strength and when I got home, I fell apart."
They are tormented by their loss and the search for the culprits. Blaming any side would be the same as condemning one of their sons.
The latest surge in Hamas-Fatah clashes has pushed civil society activists to declare an open-ended hunger strike to demand an end to the killings.