I’m so pissed! I just need to vent

I know it’s not my baby but I’m super protective of my niece. She’s 18 months old. Her step grandma (on her dad’s side) watches her a few hours in the morning during the week. Well she’s been throwing tantrums when she gets dropped off there and she’s in a really awful mood when she returns. I felt suspicious of this and told my sister she should be a little cautious. The other day when we picked her up her step grandma (she’s only like 35 years old) said that my niece hit her so she smacked her in the face. Who smacks a 1 year old in the face?! There’s other ways to discipline so I told her not to put her grimy hands on my niece again. Well my sister was venting to me that when they went camping this past weekend her dad kept spanking her for every little thing. She walked into someone’s tent so He picked her up by the arm and spanked her butt in the air and carried her to the car by her arm while she’s screaming from being spanked. My sister yelled at him to stop carrying her like that because he’s going to hurt her arm and he says “fine!” And throws my niece at her. She could have fallen and hurt herself. I’m LIVID. I know it’s not my place deciding how to discipline since I’m not the parent but it breaks my heart to pieces for my niece to be spanked like that. I want to go beat all their asses I’m so sad I want to just cry. We’re really close since it’s my twin sisters baby. Me and my sister are the main care takers for her since she usually only sees her dad on weekends because he works 6am-8pm