Some of your story sounds like typical family dynamics like your girlfriend, I always felt that my sister was spoiled and got away with everything. But the hitting concerns me. This could lead to someone getting hurt, or worse.
Find an Older Adult Who'll Intervene
Please find an older adult who can intervene. Is your girlfriend still in high school? If she is, please see her high school counselor right away to get help.
If she's not, you'll need to try to find another responsible adult who can help without angering your girlfriend's parents.
This might be a former teacher, your own parents, or someone else you can trust. The important thing to know is that you can not - and should not - handle this on your own. You could create more problems than there are right now.
They Need Family Counseling
Your girlfriend's family is a great candidate for family counseling. They don't have to be perfect like the Brady Bunch, but they shouldn't be the Simpsons either. Good luck