...once Sir Walter dies. We know that Anne can only got but a small part of the share of ten thousand pounds which must be hers hereafter. (chapter 24)

I assume that the full share of the 10,000 pounds was not available because it has been borrowed against by Sir Walter. I'm just guessing that Mary did get her share when she married Charles Musgrove (the amount originally was 15,000 pounds?) At best Elizabeth would be entitled to 5,000 pounds. That would mean only 250 pounds a year income.