I think that it is obvious that JA was saying that it was Old Mr D's stupid fondness for littlte Harry that made him leave the estate as he did. If ti was "only to be expected" that he'd leave the estate to the male descendants and leave out the females, then why would Henry D and his wife and daughters have expected anything different? Surely they'd be foolish to be disappointed, as we are told that they were, that things happened as they did, ie that Henry D did not have full control of the estate? And John D would hardly be all that bad, if he felt "well the estate's mine, of course it is. Im the male heir.. and the girls have a bit of money so why should I be expected to impoverish my son for them?"

John D already had a fortune, and was comfortalby off and so was his son.. so IMO it is clear that JA felt that in that instance (while generally the land did go to the boys) it woudl be right for Old Great Uncle D to leave John and Harry out and leave the estate fully to his nephew who, with his wife and daughters had looked after him in his old age and shown him kind attentions. JA clearly does not approve of spoiled little Harry and feels that it was stupid and ungrateful of Old Mr D to take such a fancy to the little spoiled brat and make him richer at the expense of the 3 girls who had almost nothing. Just as in Emma, when Mr Weston knows that his son Frank is well provided for as the probable heir to Mr Churchill, he feels that he can leave Randalls to anyone he likes - such as the child of his second marriage, even if it is a girl. It should have been possible to leave Norland to Mr Henry D entirely adn let him make what arrangemetns he chose, to provide for his daugthers, such as maybe leaving the estate to Elinor's eldest son, provided he took the name Dashwood.. and to save money to provide better dowries for the younger girls.