I have a similar situation in which the older child is the golden child and the younger one is the scapegoat. XAH would deny phone access so I provided prepaid cell phones. Also included a line in the divorce decree about children not being denied phone access to other parent.
I also use a counselor after any incidents. In 2 cases XAH physically manhandled younger child, in one leaving his handprint on her arm in the form of a bruise. I told the therapist and she called social services due to legal requirements to report abuse. She also spoke with the child. In my state social services did nothing but start a paper trail, but it is documentation.
I now do not allow myself to vary from the decree. At first I felt guilty for doing that but then I realized I was not protecting the kids if I gave into him and I was actively modeling an unhealthy way to respond to a disrespectful person.