Probably terrible advice I know, I had surgery planned that the hospital said would take around six months recovery time, my employer was aware of it and were very understanding, but there were others described by myself and others as "the untouchables", I'm sure every business has them, so in the week before surgery I made a point of telling them how bad there decisions were, etc. Then six months down the line walked in, explained that my absence wasn't fair on the company and resigned with immediate effect after I'd explained my time off wasn't fair on the company. I had a look round and the department had fallen apart, something I found quite funny.
It's a difficult one in your situation with family loyalty, but consider resigning and just before you leave tell those that argue what you exactly what you think. It would be interesting if they have to recruit someone new (and probably more costly) that has the same ideas as yo. This office manager sounds like a right ****!