Welcome MaggieRap: A raging parent can be very disturbing for a child. I can understand your concern.
Is the father in the picture? Can you share an example or two of when she raged at the children? Were you at her house at the time, or the reverse?
What is your in law's history, in regard to mental health? To your knowledge, has she ever been treated for a mental illness/disorder?
You may want to start with
ADOPTING A PROBLEM SOLVING MODELIf you scroll up to the very top of this page, you will find a large green band. One of the menu's there, "Tools", has links to some helpful strategy. As you have time, you might want to tackle a link at a time. The first strategy, "Wisemind" is a good strategy to master.
This is a safe place to share. We look forward to hearing more of your story. With more specifics, perhaps we can steer you to some immediate skills that might be most helpful for you situation.