One is tempted to see it in the light of a childless couple who yearn for a child to whom they can leave their riches, and whilst I don't discount the importance of such feelings, I really wonder if there isn't more involved in all this. As Lief says, Edward was there even on the honeymoon, as if this was a calculated move which had nothing to do with childlessness. Why Edward? Apart from age and temper, which made him the most appealing of the bunch perhaps, and the one least likely to shine academically, from what can be known. Was there something about the inheritance of the Knight estates that precluded them being inherited by children of Thomas Knight? Was, there, in fact, an entail on the estate that stipulated that it must go to a child of an Austen? If there was, it suddenly makes penniless George Austen an interesting marriage prospect- he doesn't have any expectations for himself, but he has great expectations for at least one son....

Is there any hint anywhere about this?

I ask this now, because the whole action seems a bit strange to me. And also becaue later in life, Edward's inheritance was contested ...but we will come to that bit later.