Hi Diane-
I'm sorry it's taken a while for you to get a reply -- it's been pretty quiet here recently.
I can't even begin to figure out what your stepmother meant by your dad eating more than she does. Could she be hinting that she needs more money? I think that, for now, I'd probably just let it go.
You said that she has used his dementia as a "weapon" against him. Could you elaborate a little? Sometimes, what may seem mean to others is what's necessary in caring for a dementia patient. The patients can be extremely difficult to deal with, and even violent, and can be very resistant to things like bathing, getting dressed, eating, etc. It's rarely productive to try to reason with them, because they aren't able to process things logically. Often, the only way to get them to do something is to be firm about it, like you would be with a stubborn child.
Especially if you aren't able to be around your father and stepmother a lot, it's hard for you to really know what she may be dealing with in trying to care for him -- it can be the most frustrating, heartbreaking, thankless and isolating job in the world.
I can't really answer your questions, I'm afraid, but I would suggest that maybe you try to observe them as closely as you can for a while. It could be that your father is worse that you realize, and that your stepmother is really suffering with the stress of taking care of him. Does she get much help with him? Of course, it's possible that she may also be developing dementia, so that's another reason to sort of keep an eye out. Spouses of people with Alzheimer's are much more likely to develop Alzheimer's themselves, so the possibility can't be dismissed.
I hope you'll continue to come back here and let us know how things are going. You might also get in touch with your local Alzheimer's Association and talk to someone there, or join a support group. They can be a tremendous source of information and help.
Best wishes.
Carol