You sound like you already have the answer. Why don't you actively pursue the new living arrangement with your cousin, and get help making that happen from your mother's family or your older siblings, and if all else fails, ask your father to get you help with a psychotherapist.
Your step-mother may feel insecure. She may not see how her actions are interpreted by you.
Although we like our parents to make good, healthy decisions, sometimes they don't. Sometimes there are other dynamics to the relationship that we just don't see.
I use to wish my parents were dead too. I don't expect it would have changed how I was feeling though. Perhaps only made me feel worse.
I think that running away from the problem doesn't make it better. You will likely find that many of your problems will travel with you to your cousins.
To me it sounds as though you feel alienated. Your mother is dead, your siblings have moved away and you aren't connecting with your father and his wife.
I'm not the best person to offer advice. Maybe accessing counselling (?through school) would be my best suggestion. It sounds like you have some tough things to work through.
I hope others can give you more helpful suggestions.
Predominantly it is about how you are feeling though so keep that in mind.