When is it enough?

So this is sort of a rant, so my sister has a boyfriend who we all hate, he’s childish and very controlling, when we first met him over FaceTime back in 2015 i knew he was a issue, he convinced my sister to get a big ass tattoo with his name on the top of her right boob, and guess where her name was at on him ? Yeah exactly no where lol, now a year later my sister got pregnant, she was about 6 months pregnant and so skinny and so stressed out looking when we seen her for Christmas. Shawn wasn’t around half of the time and also leaving without telling her anything, my sister was also hopping around from house to house because family members put her out because she wouldn’t follow their house rules like not letting her boyfriend sleep over. Fast forward to till this day her boyfriend slapped my nephew on his stomach so hard he had red bruising, and my sister did nothing but say she didn’t call the cops because she doesn’t wanna go to jail, what? Why would you be arrested for defending your child? Her boyfriend once whipped his daughter( from a previous relationship) for an hour and my sister said nothing about that, we’ve told her constantly leave and get some self respect, she cries she want to leave but let’s shawn manipulate her. I stopped caring about her relationship but when she showed us the pics of my nephew bruised it bothered me so much and made me wonder how can you be so selfish and let this man hurt your child and you’re still with him?