@talezassian I can understand your friend's concern. The younger the children are the harder it is on the woman and her children. These women receive very little support and understanding. As a therapist I am amazed at how much the dysfunctional ex partner can get away with within our legal systems. I can understand your friend's sisters concern. I have seen over and over again child custody cases with DV histories, with a fifty/fifty split, (and on occasions, full custody given to the manipulative parent), which only keeps the woman and children locked in the same, if not worse situation then before she left her partner. Because her partner still maintains their controlling and manipulative behaviours, but now they are often extremely angry, and revengeful. The women (some times men) go from bad to worse. And the children are used to hurt the partner that left them.
My experience as a therapist helping women that have come out of coercive controlling relationships that are separated (often with children), is that when they try and speak up, no one within the system believes them. They feel police, mediators, judges are more likely to believe their narcissistic ex partner, who are very good at manipulating. These people working within the system (including lawyers) need alot more education, training, and selection processes. It's a complex problem because there is often psychiatric and forensic behaviours, that they do not appear to be very good at understanding.