Princess Diana's Brother Opens Up About Her Violent Outbursts

Princess Diana's brother is opening up about a side of the late royal that people rarely heard about ... saying she got physical on a few occasions when pushed to the brink.

Charles Spencer tells People he can remember three times Diana lashed out physically ... he says she slapped their father, John Spencer, after learning he planned to marry their future stepmother, Raine Spencer.

Charles says Diana also once shoved him off his bike for seemingly no reason when they were kids ... but the most explosive incident came years later, when she allegedly pushed Raine down a flight of stairs.

Diana's brother claims the push happened in 1989, after a family rehearsal for his first wedding. According to Charles, Raine kept Diana's mother, Frances, separated from the rest of the family ... and then insulted Diana by saying she couldn't believe Prince Charles had married someone so stupid.

Charles isn't defending the violence, but says there was a buildup of strong emotions and Diana believed she was standing up for their mother.

He says those were the only three incidents he can recall ... making clear he doesn't remember Diana regularly getting physical with people.