Probably the single most important factor to examine is father or mother’s new significant other. When you have children, the unfortunate thing for many of us is that we are bound for the rest of our lives to someone who we no longer (and may never have) mesh with. That relationship can continue, in a different way, if that person allows it to. Some people do not want the relationship to end. They may no longer wish to be with you, but they also don’t want you to be with someone else. That is because many relationships are about imposing control over the other person, and not about anything else.
The only worse than this kind of person is if that person finds another person like them. Two people with personality disorders can have a synergistic effect in that individually, these people might be trouble, but together they spell a disaster for the family, and yes, even themselves.
The new girlfriend now sees mother as a threat that must be destroyed, rather than a potential ally in the difficult new life each of them face. Unable to achieve cooperation, they resort to an all-out attack upon the other party. If anyone warns you about the new significant other of your ex, take heed.