Well, BIG UPDATE:
This entire dilemma is now a mute point.
After days of just chaos and my SIL being completely miserable, she finally called her husband and had an actual conversation with him about things.
Turns out it was more of lack of communication on their part which led to lack of help and understanding than it was abuse I am happy to report.
SO! She packed her stuff up and left the next morning to head home.
I know it was the right thing to do... her entire demeanor changed instantly. She really didn't want to leave her home, her husband or her life.
To make sure things ARE ok, my husband and I will make sure to keep in contact with her frequently to make sure she is doing what she promised us all she'd do to improve things and not be so down.
But once again it was very obvious that our Amelia likes to be the lone, alfa dog.
We do have Carmen, our other grey, but she allows Amelia to reign and it's pretty harmonious with them.
Thank you all for the advice. I truly enjoy this community.