How can I help in a child abuse situation?

My girlfriend's sister (Jessica) has three children, 1 an adopted 14 year old girl (Lindsey), and 2 9 year old twin boys. She treats the girl w/ extreme emotional neglect and abuse. The girl is constantly ridiculed for being stupid, she is chastised and called a liar for the smallest of infractions, she is made to do ALL of the dirty work around the house, she is gotten out of bed if a mess is made in the night or if it is found out that she did something wrong. The girl does not know that she is adopted, and it is the general consensus of everyone that there is something wrong with her mentally. When her IQ was tested as a small girl, she rated as barely above mental retardation. Her biological family is not the best gene pool to dip into. Anyway, Jessica has an obsession with "discipline". She feels that she was not adequately disciplined as a child, and that is why she did drugs and partied and dropped out of school. Yet, she says her mother hounded her too much and that's why she left. Well, she is always saying that she is going to make the girl's life a "living hell", and she does. She is currently forcing the girl to eat only spicy ramen noodles. She found out that while she was gone (out doing something with the family while Lindsey was at home alone), Lindsey had eaten ramen noodles and tried to hide the evidence. The trash was at the bottom of the can, and the bowl was rinsed out. She found out b/c there were noodles in the sink. There is no telling how long Lindsey is going to have to eat noodles, but I witnessed it tonight when we had Krystals and pizza. Everyone (except my girlfriend and I) was laughing at Lindsey and the boys were asking how it tastes. The "stepfather" was saying that she should make her eat something really nasty, like brussels sprouts or asparagus. Jessica is always humiliating Lindsey in different ways. One time, Lindsey was hopping in the house, so Jessica made her hop laps around the house. She does mean, spiteful things all the time. It is Lindsey's sole responsibility to take care of their 3 dogs, including going out in the middle of the night down the hill in the woods (they live on a mountain) to bring one of the dogs in. Recently Lindsey was telling me about how she was scared coming up "the other" hill. I asked, and it turns out that while Jessica was gone, she made lindsey walk by herself at night about a tenth of a mile down road (with big hills and turns, i would have been scared) to get a note off a door. This is NOT a safe situation for a 14 year old girl. Jessica is weird, in that she gets furious if anyone tries to take up for Lindsey. She tried to throw my gf down the stairs one time b/c she tried to take up for Lindsey and said that Jessica is a bad mother. She always talks about sending Lindsey back to her bio family, who are PURE garbage. She says these things in front of Lindsey, yelling and screaming about what a horrible little girl she is, how she's a liar, etc. Granted, Lindsey does have a problem about lying. But Jessica will often scream at her all day, getting up in her face like a drill sergeant (Lindsey is tiny), and from stories I've heard, she has hit her, including heading. She *NEVER hits her though. Just the other day, though, she apparently beat her with a spatula and belt. She treats her boys like gold, and they have absolutely zero responsibilities. One of them has horrible acid reflux, and puked all over his sheets last night. Lindsey was made to get out of bed tonight to bring the sheets down and clean them. Lindsey cleans the bathrooms almost every night, often up cleaning long after everyone else has gone to bed. She does most of the kids laundry. She is also failing in school, and Jessica refuses to help her or get her help. She says that Lindsy should pay attention and that she is too stupid to learn anyway. Lindsey is not allowed to have any friends, and is extremely immature. She only hangs out with her two brothers. She has the mindset of a 10 year old, and Jessica calls her out for that. Tonight she was screaming at her b/c she had a marker out on the floor because she does a bunch of art things in her room and writes songs and "stupid make believe childish sh*t like that". The girl is locked in her room most of the time, and that is all she has. She is often left home by herself while we go out to do fun things. I don't feel it is fair or safe to leave her there for such long periods of time. Lindsey apparently misbehaves and teases other children at school, and I cant help but think that this is because of how she is treated at home. One night I witnessed the boys come in and hug and kiss their mother and "stepfather" goodnight. About 5 minutes later Lindsey walked in. "What do you want??" "i was just gonna say goodnight and get a hug..." "Well, wait." they went out to smoke and Lindsey just went to bed. I