I don't see how that could possibly work. Do you mean to get income off the estate, like an annuity, or to actually live there? What if one of them lived to be 80 years old or older?

Harry is only 9 years younger than Margaret. So imagine an 80 year old Margaret and a 71 year old Harry, now the owner of the estate, with children, grandchildren and great-grandchildren of his own to provide for and to consider in his own will, but there is old Aunt Margaret still drawing an income off the estate?

I agree he probably didn't expect his nephew to outlive him by only a year. It would have been nice if he had done more for them, but it's not really very surprising that he didn't. And it's not because little Harry was cute. It's all about the family legacy, pride and securing that estate for future generations of male Dashwoods. The fact is, it just didn't matter to him whether his nieces married well or not because they were girls, and girls were not that important. We don't know why the old uncle's own sister was unmarried, either. Maybe he never bothered about providing a dowry for her?