It seems the one thing you can't do is wait. If he's got terminal lung cancer things can go downhill fast.
I can't advise you and I'm sure any lawyer would disagree... but if it were me, I'd knock on the door and inform my step-mother that I'm going in to talk with my dad right now... whether she likes it or not. Then I'd go in. I would be careful not to do or say anything that could be construed as a threat, but I'd go in and have a chat with dad. It might not be a bad idea to have a recording device running the entire time, in case you have to defend yourself later against whatever she claims you did/said.
Another option, that might work better, is to show up at the door with a few friends/family members. She might back down more readily in the face of a crowd.
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