In a battle between the Namibian champion and his Japanese challenger at the Waterfront Hotel in Cebu City, Philippines, the 22-year-old Kameda (28-0, 18 knockouts) used his superior speed and technique to outbox Ambunda (20-1, 10 KOs) over 12 rounds
The scorecards of the ringside judges all favoured Kameda by 118-110, 116-112 and 117-111. Kameda appeared to drop Ambunda just before the final bell with a short right, but referee Raul Caiz Sr. ruled it as a slip.
The fight was Ambunda's first defence of the title he had won from Thailand's Pungluang Sor Singyu in March.
The eldest brother, Koki, 26, was the first to win a world title in 2009, winning the WBC flyweight belt before losing it in his first defence. He is the current WBA bantamweight titleholder, having made seven successful defences.
Daiki, 24, won the WBA flyweight title in 2010 before losing it a year later. If he can defeat Rodrigo Guerrero next month in Kagawa, Japan for the vacant IBF junior bantamweight title, all three will reign simultaneously.
Tomoki Kameda also made another bit of history on Thursday, becoming the first Japanese boxer to win a WBO title.
There was another title change over in Bangkok, where Koki Eto made it an even better day for Japanese fight fans.
He emerged victorious from a 12-round war with interim WBA flyweight titlist Kompayak Por Pramuk with a unanimous decision - 114-113 x 2 and 116-111 over the Thai veteran.