The Jane Austen system makes sense though, if you see the adoption/fostering as breaking the link between the birth parents and original family. The relatives that adopted them ARE the family, therefore the biological mother and sisters are just relations. (I am not saying he wasn't a twat to act like that).
I know of quite a few sub-saharan African cultures have similar set-ups, where its quite common still for better of (usually but not always) relatives to foster children and give them lots of advantages. The issue there being that even though the child is being brought up by other people they are still the original child of the first parents, with the usual obligations/expectations on them. One of the reasons why international adoptions can cause so many issues is that the original parents are going into it with one set of expectations (and the concept of adoption severing birth ties just doesn't exist in their culture) and the adopting parents going in with their own ideas (adoption severs the ties, the new parents are the parents). Causing confusion and heartbreak and also issues for the adopted children down the line (eg going back to Africa to meet their original family and being expected to buy cars etc for their parents like a good child would and everyone having their feelings hurt).
I know its off topic, but a lot of profit making adoption companies were fully aware of this mis-match - meaning effectively that they knew they were misleading the parents. Now we have profit making surrogacy companies and we expect them to navigate incredibly difficult issues with perfect honesty and sensitvity. Plus ca change.