well it almost seems like there is very little to add here! Obviously where there has been any abuse, there will then be a desire from the victim to avoid such situations again.
Sometimes to the point of hyper vigillance.
In the case of your sister and brother in law, its a sad situation, where everyone looses.
I would just say that if there was alot of conflict in their relationship post seperation, maybe she was unable to see any improvements in your brother in law, because of the conflict.
Thanks dragonfly, its a good post and it makes us think, which is most important of all!