The 21-year-old then cheered from the sidelines as her big brother beat Georgia's Vazha Margvelashvili in the under-66kg gold medal fight.
It made them unique in Olympic history as the first siblings to win gold medals on the same day in an individual sport. Both are also two-time world champions.
"We had a strong determination, that's the reason," Uta said. "This was our dream since childhood."
Hifumi said his sister's success inspired him to victory.
"I was sure she was going to get that gold medal," he said.
"At that time (she won), I got even more determined to win myself. I had a fighting spirit surging in myself."
The Abes gave Japan their third and fourth gold medals of the day after Tokyo native Yuto Horigome become skateboarding's first Olympic champion and swimmer Yui Ohashi won the women's 400m individual medley.