Justin, 27, and Sydney, 30, are the children of Simpson and his late ex-wife Nicole Brown Simpson. Nicole's sister Denise Brown is Sean's mother.
Seated among other guests during the stunning outdoor ceremony at The Resort at Pelican Hill, the Simpson siblings were "very chummy" with their aunt, a source told E! News.
"Justin was in a celebratory mood and having a good time," the source added.
Sydney was 8 years old, and Justin only 5, when their mother was killed with waiter Ron Goldman outside her Brentwood home in 1994. O.J. was subsequently charged, then acquitted of the murders – a court case that is the basis for current TV series The People v. O.J. Simpson: American Crime Story.
VIDEO: How Well Do You Know the O.J. Simpson Murder Trial?
Both Sydney and Justin – who also have two half-siblings, O.J.'s older kids Jason, 45, and Arnelle, 47 – have stayed mostly out of the spotlight throughout the past two decades.
Saturday's soiree included fireworks and a live performance by singer Sara Bareilles.
Reinhardt, who got engaged to Brown last August, wore a lace ball gown and flower crown for the lavish affair.
The bakery owner appeared on MTV's popular reality show during its second season, and, later, on its spinoff The Hills while her brother, baseball player Doug Reinhardt, dated Lauren Conrad.