Lady Catherine says, "... I see no occasion for entailing estates from the female line. It was not thought necessary in Sir Lewis de Bourgh's family." I have always understood this to mean that Anne De Bourgh will inherit Rosings.

I do agree that it seems unfair, but it was certainly not illegal. Even if Longbourn estate had not been entailed, it doesn't mean that all daughters would inherit equal shares of it, however, and that might seem unfair as well.