Long story, but my DP's father no longer lives with the family and has not done so for many years. His 2 sisters are half-sisters from his mother's previous relationship, and are both about 7 or 8 years older. He feels he has never really got on with the eldest sister but put that down to jealousy as he is the youngest.
The other week, his halfsister and I got a bit giggly on a few bottles of wine and I asked why she had never got on with my DP or his father. She revealed to me that it was because when her mother and DP's father got together, he used to hit her, and her mother - proper beatings along with threats of "if you tell anyone, I'll kill you". I was obviously shocked and agreed to keep this secret as it was clearly an emotional point for her.
DP has no idea that this is the case. I now no longer want to spend any more time with his father at family events, and he's beginning to notice. Do I tell him that it's because his dad was an abusive arsehole? I know it's not my place to reveal this sort of sensitive information but I'm really not sure what to do.