Worried for someone...

My SO'S mother doesn't choose men very well. Her and her husband of less than a year are fighting.

This man had an abusive past and I'm not only worried for his mom, though she can handle herself well, I'm mostly worried about his sister.

She's 12 and has been very very hesitant around her step dad as it is.

But now that her mom and him are fighting, she's been really weird. She called us when we were asleep one night and the message was, "They're slamming doors and I'm scared. I don't know what's going on."

Now she can be a bit dramatic, once when my SO and I got on a slight argument she freaked out and wouldn't talk to us.

But it still worries me because she is so unstable and scared. I want her knowing our house is safe, and she's within walking distance. She's tried to commit suicide before. I'm just so worried about this girl. What can we do to not get into the fight but still support her and keep her safe while they most likely are walking down the path of divorce?