I have always read the hour and a half to dress as JA's way of subtly hinting at their indolence. But I now think that maybe they took an hour and a half on purpose so as to not be around Elizabeth and Jane in the sickroom. As JA shows them to be self-involved I can't imagine them wanting to dirty their hands in the care of an ill person. Also, to be away from the Bennet sisters only allows them to denigrate them in their awful country ways . ;)