Should I do something as a Aunt? :/

My sister in law called me today that her and my brother-in-law’ were fighting very bad. She also said that she was out doing some stuff and that when she got home my brother in law had left my 2 year old niece alone in their apartment!! My heart broke because what if something would’ve happened,??

My sister in law is mad but wants to fix things with my brother in law. I just care about my niece and it breaks my heart to know she was alone for who knows how long! I told my SO and he says he doesn’t believe his brother would do such a thing. But he did do it because my sister in law didn’t have keys to her house and had to call my mother in law to bring the spare. I don’t know my only concern is my niece. I don’t know what to do.