And really,none of the Ward sisters were very rich in the first place.

"About thirty years ago Miss Maria Ward, of Huntingdon, with only seven thousand pounds, had the good luck to captivate Sir Thomas Bertram, of Mansfield Park, in the county of Northampton, and to be thereby raised to the rank of a baronet’s lady, with all the comforts and consequences of an handsome house and large income. All Huntingdon exclaimed on the greatness of the match, and her uncle, the lawyer, himself, allowed her to be at least three thousand pounds short of any equitable claim to it."
Maybe Sir Thomas regretted marrying the "indolent" and non-wealthy Lady Bertram, and wanted to make sure none of his sons did the same.