I think I read somewhere, someplace, sometime, that one of the reasons Jane Austen has Sir John charge the Dashwood ladies rent, albeit a minimal rent, is to respect the 'independence' of the ladies - which would mean he would not have the moral responsibility of finding suitable husbands for the girls. If he did not charge them any rent, he would have effectually become the male head of the family and would have been morally obligated to ensure the girls were taken care of and married well.