Sorry, I don't have any suggestions on how to get this person to stay. But please don't think about killing yourself. In a few years time you will be moving out yourself and things will be better for you. I know a few years may seem like a long time, but you have your whole life ahead of you, and if things are this bad now then they can only get better. Take and good luck.
Don't kill yourself it sounds like a good idea but no because if they are the type of person they are they will not change anything after you die. My advice: Mail them a letter telling them what you told us.Sounds stupid, but it worked on my brother. You know we really need an advice collumn webmaster!
Please post an update - you have a lot of people thinking of you.