It was a clash between Lorenzo, 29, and 26-year-old Roberto, as well as former AC Milan and Italy teammates Filippo Inzaghi and Gennaro Gattuso, now coaching the two clubs.
"I'm very happy, but sorry my first goal in Serie A was against my brother," said midfielder Roberto, who opened the scoring after half an hour against his hometown club who he left two seasons ago for Benevento.
"Playing against my brother and Napoli, the team of my city, is a great feeling, but we are all a little saddened by the result."
Lorenzo Insigne marked his return from injury with a stunning equaliser for his 68th top-flight goal for Napoli.
The Napoli captain also had a goal ruled offside before blasting in with his weaker left foot after an hour, with Matteo Politano setting up Andrea Petagna for the winner seven minutes later.
"After the game? I told him (Roberto) my left is better than his," joked Lorenzo.
"No, I'm kidding, we also hugged because today was an exciting day for our family. I'm happy with the goal and congratulate my brother too.
"The left foot is not my favourite but I struck it very well and I must say it was a beautiful goal."
Gattuso's side got back to winning ways after their surprise 1-0 Europa League home defeat by AZ Alkmaar, moving one point behind leaders AC Milan which hosts Roma on Tuesday (AEDT).
Promoted Benevento is 12th after its third defeat in five games.
In Sardinia, Argentine striker Giovanni Simeone, son of Atletico Madrid coach Diego Simeone, scored his fourth goal in five games as Cagliari beat 10-man Crotone 4-2.
Five of the goals came in a thrilling first half. Cagliari is 10th with Crotone bottom of the table.