I just don't think this is universal. Even if he didn't hit her, their issues were always unresolved and their situation never got better, even after counseling.
It's anecdotal at best.
Not only it can't be generalized, at different ages the outcome may be a lot different.
Kids need a certain level of maturity to understand properly conflicts and learn conflict resolution otherwise they simply feel them as a crack in their world.
There are not fast and hard rules here, every kid/situation may be quite different...