He might even have recognized some of his own behaviour as a child in the little fellow. When the Henry Dashwood family arrived at Norland, Margaret would have been about 2 years old, with Marianne 6 & Elinor 8; they might not have been noisy children, and even if they were, that behaviour should have been outgrown before Harry came along. Harry could interest the old man just by being different, in addition to being a male heir to whom the property could be passed undiluted.

And I also don't think "He meant not to be unkind" is necessarily ironic (dripping or otherwise). I assume that old Mr Dashwood had no reason to believe that his nephew Henry would die in the near future; if the 1000 pounds for each great niece were invested at 5% interest for 14 years the lump sum would double to 2000 pounds each. AND don't forget that Mr Henry Dashwood had a life interest in half of his first wife's (which was large) -- even if he couldn't touch the principal of that, the income should have been at his disposal, to add to the girls' funds. And the older Mr Dashwood probably assumed that the Miss Dashwoods would be married within 10 - 14 years, so they mightn't actually need more than the 1000 pounds (plus interest).