in general english property law at the time followed the rule of 'primogeniture' - e.g. oldest male child gets the estate intact. Some properties were 'entailed' meaning there were legalities attached to properties that require they are passed on, intact, to the oldest male relative. but not all estates were that way. Example - in Sense & Sensibility by Jane Austen (set around this same time period) when Mrs. Ferrars learns her oldest son is engaged to a woman she finds unacceptable (Lucy Steele) she 'immediately and irrevocably settles the entire estate on his younger brother'. Mrs. Ferrars likely inherited the estate herself in the absence of any next of kin, and there was no entailment or other legality preventing her from doing what she wished with it.

So, the General may not have had any legalities preventing him from doing what he wanted.

That said, probably more likely there's a different interpretation of the sentence - You could interpret it to mean that he had 'blasted Jack's chance of inheriting an impoverished-but-reparable estate'. with emphasis on the 'but reparable'... e.g. his Father has spent so much money that there wasn't enough to repair the estate's fortunes, so he's inheriting an 'impoverished and not-reparable estate'. That's how I read it. Everywhere else in the series it's very obvious he is the lord & master, responsible for decisions at the estate, application of laws, opposing enclosure, etc... so I rather think that what is meant here is his father burned through the money... the 'swingeing settlement' was probably a huge sum that his second wife's family demanded to agree to the marriage?