stress of in laws!

there is always screaming at my in laws and I feel that my sis in law who had our niece 2.5 years ago and hid it for 8 months is suicidal and she and I are close and I love my niece so much and she doesn't have a father in her life and my sister in law gets screamed at all the time and her parents call her fat and a bad mom and good for nothing and I try so hard to be there for her and listen and make her laugh and help with her daughter but I can't cross certain lines because I'm not blood or a disrespectful person and I have a gut feeling something is going to happen. I am close with my nieces father and he wants to be involved but my In laws won't let him see her but once a month. I need advice please help