I am so sorry you are going through this and wish I had some wonderful words of wisdom for you.....but I dont.
We have never had any dominance issues with our crew (thankfuly). I know you must be scared for Mandies safety though as her brother is a much BIGGER brother. I really hope you can figure this out. Sending thoughts and prayers your way....as if you need more stress in an already stressful time!! Hugs!!!