What can I do about in law that left 6 month old baby home alone?

my brother and i was gone 20 min came back unexpectedly and found out his wife had left their baby home alone. She claims she just ran down the road and was only gone a minute when i counted at least 5 minutes while my brother was inside before she came home. I told my brother to bring the baby and to leave her and he got mad at me and then later they claimed it was not that big of a deal. How many other times has this happened or will happen. Do i call the police or DHS or encourage them to go to counseling. It just makes my blood boil I also believe there is abuse going on meaning shes the abuser she seems pretty nutso. I am at lost i dont want to be a betrayal to my brother but i fear for the safety of the baby