In my honest opinion, with today's economy, I'd just suck it up, or ignore her.
The fact that you're getting a free room, and have to pay no rent, and you're over 18 seems like a good enough reason to just ignore it. You said you can't really move out and live on your own because of the economy, so the only advice I can give you is to suck it up.
You can be in FAR WORSE situations then dealing with a step-mother. From what I've read, none of the examples you provided are that bad. Shit could be worse bro, just suck it up for now.
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