Very easy to see from this thread how so many women end up in relationships which are violent.
As daughters they were told THEY were at fault if they were pushed and shoved in the home they grew up in.
Their violent partners had parents like the OP and her husband who accepted being told by their thug of a son that it was "sorted" when they assaulted their sister and refused to explain themselves.
Possibly drugs are involved.
Whatever.
He has assaulted his sister and I have no doubt will assault someone again.
Siblings can roar at each other and piss each other off.
It is not normal behaviour for that to escalate to assault.
But the type of families that accept that are far more likely to have children that will end up in abusive relationships, either being the perpetrator or victim.
It says absolutely everything IMO about how the two children are viewed that the OP would accept him refusing to answer her questions and telling her it is sorted.
I so hope the daughter has supportive friends that she tells about this.
If one of my daughters told me her friend was in this situation, I absolutely would offer her a safe place to stay as her parents priority is clearly their violent son.
Golden child/scapegoat dynamic comes to mind.
How any mother could hear that happened to their daughter and that she was crying on and off all day and be anything other than devastated for her is beyond me.
Accepting "its sorted" from her son just tells me everything.