When a marraige couple is in a domestic dispute which turns into a physical altercation do outsiders have rights to interfere w/o going to jail?

My son and wife were in an altercation in which the wife left with their child only to return back to the residence in which resulted to an altercation. The wife's mother and boyfriend shows up and the boyfriend who is law enforcement hits my son and they start fighting. Why both individuals were not incarcerated together, because the mother's boyfriend was not a residence at the establishment? Why would the wife return home for only 5-10 minutes before her family comes through the door? This incident does not set well with me as if this was a plotted case? I need help in understanding this and needing to find the right attorney for my son. Domestic violence I do not condone, but my son was also treated unfairly in this as well.