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First, this letter is not for real - it's a writing project. If this situation were real (not impossible, just improbable), then Cary's advice would be sound: tell Dad to get it together and
move on since you're not ready to take on this responsibility.
But there is one more possibility. Make a pact with the kid and, if she's amenable, agree to take job. Four years of step-child finishing school in exchange for a life of wealth and leisure. Not a bad trade off.
Sit the two of them down. Lay it out. Marry Dad. Educate the Kid. It's a four year project, and at that point everything gets renegotiated. If you succeed, great. If you fail, everybody goes their separate ways.