How do you know if you are being manipulated?

My 79 yr old father just lost his home due to flooding from Hurricane Sandy. He had been living on his own with my cousin, who has elder care experience, coming in an hour or two a day to check on him, help with housekeeping, run errands. He has Parkinson's and a number of other issues. He is devastated by the loss of the home he grew up in, of course. He is currently staying with my sister and her husband in their very small house. My house is larger, but I am still without heat from the storm. He is driving my sister crazy. He will not make decisions or do anything for himself. He will not bathe or shower, answers, "I don't know" to everything he is asked. we know he is grieving, but he seems to be pretending to be much more helpless than he was before the storm. We have found him an apartment in his neighborhood but out of the flood zone, but he says he wants to stay in my sisters house, which is impossible. He has always been a difficult man, but my sister is fearful that he may be heading into dementia because of his unwillingness to do anything for himself. On the other hand, people outside the situation have suggested he is manipulating us. How do we know? Please don't suggest a medical work up. We have yet to deal with a doctor who doesn't respond to concerns about our father with, "Well, he's old."