It’s sadly common that when an adult child cuts off a parent, he or she stops talking to siblings, grandparents or other dedicated friends or members of the family.
Why do they cut off everyone and not just the parents?
In general, it has to do with the psychological fragility of the estranged child: If you’re very confident in your beliefs or perspectives, you can tolerate views that differ or diverge with your own. You can hear a sibling say, “I don’t think Mom’s that way.” Or, “We were never abused.” Or, “Sure, they were far from perfect but they did a lot of things right, cut them some slack. They’re your parents.”
A healthier person would be able to hear that perspective, perhaps get annoyed, but agree to disagree with the person who’s stating it, whether it’s a sibling, aunt, uncle, grandparent or family friend. A more fragile person develops a more black and white construction of reality because he or she is too easily influenced by others.
For the same reason that they may be vulnerable to getting into a cult-like relationship with your SIL, DIL, or a real cult, they are unable to hold onto their own perspective and can’t tolerate the grey.
This also explains why those adults who are psychologically fragile have to reject the parent in order to find themselves.