My gfs brother is abusing dogs

Location: Indiana.

Hey I 21m visit my gf 22f at her home quite often we’ve been dating about 8 months and we have a bit if an issue. Her brother I’ll call him Sam lives with her as a room mate he pays 2/3rds of the rest and my gf pays 1/3
Sam has insaine anger issues and he has a dog a German shepherd named Roman whenever I come iver i kinda have to rush to her room bc since Sam abuses the dog roman sees me as a massive threat and barks like crazy causing Sam to come out of the room and start yelling at the dog. I’ve never witnessed him beat the dog only heard sounds. Him yelling,the dog yelping,sometimes the sound of something being thrown. But I feel like I wouldn’t have a solid case since I haven’t seen him do it only heard it. I do wanna state im fairly bigger than Sam and in ANY other circumstance would beat the shit out of him. However me and my gf are certain that would end with him having my gfs mom kick her out. For that same reason she’s also afraid of calling animal control. Aswell as she’s worried they will take her cats( i do need to state my gf treats her fur babies like royalty she us the embodiment of a great cat mom) so i feel helpless i feel like no matter what we do it will come back to bite us in the ass.so would i have a solid case? I have video of the garage that the dog is kept in it is revolting.but im not sure if that would be enough. Please, any advice is appreciated.