“I don’t like you”

So I need some help, advice, anything really at this point. I have a niece who is about to turn three years old. Just before she was a year old I got temporary custody of her because my sister was in an extremely rough patch and using heroin. I feared for my nieces safety so I called the police and cps got involved. My sister got intense treatment that she put herself in and proved herself worthy of being a good mom and got custody back. I was extremely happy that they would finally bond because our mother had raised her the first year of her life pretty much.

Fast forward to now and my sister has a new baby and is with a new guy, they live together in a nice house and he makes great money blah blah blah. I have always gotten a strange vibe from him, he’s very controlling and doesn’t let my sister leave the house except to go to the store and she has to FaceTime him to prove she’s there. I go there almost every other Sunday when my niece isn’t at her other grandparents house for the week and we swim. She always acts scared of my sisters boyfriend, if he swims towards her she tries to get away as fast as she can and uses the phrase “I don’t like you!” Or “I don’t love you!” I’m sorry but what three year old just says that all the time if nothing is wrong?

Her biological father and his family are Hispanic and my sisters boyfriend will mess with her a tease her and say “Leighton do you wanna go to Mexico?” Meaning his family. It pisses me off BAD. He scolds her for no real reason and when she was scared to sleep in her room he went in there and shut the door and slept on the floor.

What I’m getting at is I’m freaking concerned and I have a knot in my stomach and a gut feeling that I need to say something or do something but I don’t know how to go about it. I will jeopardize my relationship with my sister all day long if it means my niece is safe and isn’t scared.

Who do I call or go to? Any advice is appreciated. And please no harsh comments, aside from what I’ve said she lives in an extremely clean home and is taken care of physically as far as clothes, food etc but what worries me is her emotional well being.