Hey all, hope this is fine.
So, I believe my girlfriend of a year and a half is in a rather unfortunate situation.

Here's some things to know:
-I'm 18, just graduated High school, and am pursuing college. I am a fairly quiet kid who enjoys doing calligraphy and fountain pens.
-She's 16 right now.
-Her actual mom left when she was 4. She recently got in contact with her
over facebook.
-Her dad has been in many, many, many relationships.
-Her current stepmom is 33, and her dad is 45.
-She is her dad's only child.
-The stepmom has 2 children, a girl (10) and a boy (14).

The situation:
--Her whole life, she has been a very smart, shy girl. I can only really tell about the last year and a half, so I'll mainly discuss that.
--Her dad draws a check from her (Unknown amount). He will not disclose any info about the check. It is also said that he lies when filing documents so as to get more money. They are not poor in any way.
--The Stepmom works at a small candle place, and has recently been working much longer hours despite higher-paying job offers.
--Both parents are very cheap, and will only buy things on coupons. Despite this, they take regular vacations. The dad even bought himself a new speedboat and BMW. When it comes to the kids, they are "cheaped out" on. ---This is especially true for my girlfriend, as she receives the least favorite treatment of the household.
--Compared to my girlfriend, the stepchildren are very spoiled.
--The stepmom also sometimes makes remarks about my girlfriends real mother, which really angers and depresses her.
--Both parents are very controlling, and have not allowed her to go anywhere with me without supervision for the entirety of our relationship.
--They often spy on us while I am at her house, we are not allowed to shut any door or escape their line of sight. This was very bad at one point; My GF and I simply went into the bathroom together so that I could demonstrate the cleaning of my fountain pen. When asked why, I asked if the stepmom would like to come in and observe, as she often does. She threw a fit, stormed out of the house for an hour, then came in and lectured us on her "rules". She later told my girlfriend that I was only there to "get in her pants" and that she did not like me because of my "smart mouth". I was in complete shock upon hearing this, as I manner myself very well when over there.
--The stepmom also influences my GF's dad lots; Oftentimes it's her way or the highway.
--The stepmom has a very smart-alek attitude, and tells us what not to do (a huge list of ridiculous things) based off of what she did when she was young.
--both parents do not ever seem to really listen to any of the children. They also try to push any financial expense of my GF over to her grandparents, who also are tired of paying for things. (Remember, the dad draws a check for her!)
--None of my GF's family gets along with each other, there is constant drama.
--I have a strong suspicion that the stepmom, and in part, her dad, do not want the best for my girlfriend. They NEVER consider her in anything, and always speak negatively of her despite her being the best-behaved person in the house.
--When offered a job at a place she really, really wanted, my GF asked her dad, who said no. A few weeks later, her stepmom (who influenced this decision) told an employee of a place my GF does not want to work that "She's looking for a job, Y'all should take her!" and looked at my GF with a snarky look. It seems as though Both parents did not want her to have a good part time job. This is especially bad because since her dad refuses to pay for almost everything for her, she wants a good job, but is declined and taunted like this.
--My girlfriend is constantly crying and feels low about herself because of the never ending negative treatment at her house. She dreams of going away; never seeing her parents again.
--There is plenty more, but this will perhaps give an insight into what's going on.

I would really like some thoughts and some advice on what to do. Her grandparents are also very concerned but argue a lot as well.
Thank you!