Wow.. I'm sorry to hear about all that you've been through, and what you're currently going through. I really don't think I know the "right" thing to say.. but I feel I need to say something, to try to help in some way...
It sounds as though your family is jealous of you.. of the love that your grandparents have for you, and for the things you will possibly inherit. Jealousy isn't easy to overcome.. the jealous person has to first realize they feel that way.. then want to make it better. They don't sound like a rational group of people.. not people you would accomplish anything by sitting down and talking with.. so I really don't know how to advize you on that....
What do you mean they are "litterally poisoning" your gran? If they tamper with her medication or add something fatal to her food or meds, maybe you can catch them in the act with a hidden video camera? Sneaky.. but effective if you can pull it off.
Are lawyers involved in any of this? Sounds like they should be.
Does anyone help you take care of your mother and gran? Maybe you can share the responsibilities with another family member. That will give you some time to get a job. Then you could save money for your own apartment. could you possibly live with a friend until you have some money saved up. Even living away from home, you can still care for your family.. you would just have to in shifts, with another trusted care-giver.
What does your Gran have to say about all this, about how they treat you, and talk to you?
Is there anyone you live with or know that you trust and would help you out?
It's too much responsibilty for one person to take on herself, not to mention all the added stress with the problematic family members. I think you need outside help. Do you see a counselor? I ask because you mentioned suicide attempts, death of loved ones, agoraphobia and sexual abuse. Those alone are tramatic experiences.. put them all together, its almost intolerable. You would probably benefit greatly from talking and working with a professional.
do you go to school? If you can't afford a doctor, the school would have a counselor you could talk to. They would also have better advice to give regarding your current situation.
I doubt I've helped any, but I hope I did in some small way.
Best of luck....