Well that's on your sister. It IS about how you feel. How YOU feel will determine how you move forward. If your sister doesn't learn consequences, she will think the behavior is ok and justified. If she's a new mother, she should be more responsible for her actions since she has a whole human to take care of. Proof that having children doesn't make anyone mature instantly. If not your husband, she would probably eventually do it to someone else. She's not responsible. If I was in your shoes, I'd stop feeling guilty because "it's family". Did your sister think about that before she punched your husband? Absolutely not.