I'm worried about These Children?

Okay, this question is a little long, but very important to me! My fiance has a 2 year old nephew and a 1 year old niece that I often babysit for. Their father works retail 6 days a week from about 10 am-11 pm, and their mom works as a nurse two or three nights a week from 5pm-7 am. When their mom is home, she is constantly sleeping. I've showed up at the house and heard the baby in her crib screaming and the 2 year old blocked in his room with 3 baby gates crying for his mom. She makes comments about how they are so tired, and will put them down for a 5 hour nap at about 3 pm. Do kids really sleep that long? Sometimes when she is sleeping they "get loose" and do things like pull the keys off the family laptop, eat change, and fall off the couch. When she is awake, she frequently screams at them because she doesn't feel like getting up. She doesn't swear at them, but she is constantly yelling. She also has a habit of sitting them in front of Elmo or Sponge Bob for hours and hours at a time while she sleeps on the couch or sits outside smoking/drinking. What can I do to help this situation? Their mom absolutely refuses to work during the day instead of at night, so she can sleep normally. She claims that she just isn't a day person. She got really mad when I questioned whether she was possibly depressed or had some kind of sleeping issue. Also, despite the fact that she can afford cigarettes and beer, she always claims to be broke and never pays me. I simply can't devote even more time to the babies when I am not getting paid. I need to pay my own bills! But I really care about these kids, my fiance doesn't seem to know what to do, and her husband just doesn't seem to care.