HELP! My boyfriends sister is a and I hate her? Is my relationship worth it?

My boyfriend and I have been together for 3 yrs, he recently moved from an apartment with his friends back home with his parents until we find a house together. His sister is 27 (we are 23) and she has a 3yr old kid whom she is neglectful and verbally/emotionally busive towards. My boyfriend is ignorant of the situation, but I know it kills him inside. His sister had this child by having in exchange for drugs, which she told me one night when she was intoxicated, but my boyfriend is basically in denial of this. She is living at home with her parents and they take care of the kid while she boozes and parties. She hooked up with an exboyfriend not too long ago and shortly before she also went to Vegas with me and some of my friends and she slept with two different men. Now she is pregnant and claims it is the ex-boyfriends, and she is supposedly moving in with him. He is also a very wealthy stockbroker, and I'm pretty sure she's doing it to get her ticket out of the parents house..