Repression

You know that subtlety is going to be required here. This information can help bring your family together, but you have to do it right.

You attend your classes as usual, eventually school ends and you see your sister. You immediately reveal this information to her. She looks very upset, yet not entirely surprised. This certainly isn't the first time he's done this. She just keeps forgiving him, and taking him back. She was planning on staying with him to get away from Mom, but now she's conflicted.

You explain what happened with Mom after she left. Your sister looks skeptical, but knows even though you're a bit emotionless, you've never lied to her. Plus your latest deed has put you in better standing with her. Your sister agrees to give getting along with Mom a chance.

You both go home and wait for Mom to get off work. Mom arrives, she's a little more calmer, but you know this is a volitile situation.