OJ Simpson's Former Nephew Dodges Domestic Violence Bullet

12/28/2011 9:46 AM PST

OJ Simpson
's former nephew will NOT be prosecuted for domestic violence because the D.A. is uncertain who started the fight at his home yesterday ... TMZ has learned.

TMZ broke the story ... Sean Brown -- the son of Nicole Brown Simpson's sister, Denise Brown -- was arrested along with GF Amy Johnson, after the 2 got into a physical fight. When cops arrived, both had visible injuries.

But the Orange County D.A. cannot determine beyond a reasonable doubt who started the fight and who may have been using appropriate self-defense, so both Brown and Johnson are home free.