..."puppet heiresses" (felicitous phrase!) like Miss Morton were quite common, given the life expectancy in JA's time, hence the literary tradition (not exclusive to JA, naturally), of orphaned characters.

A more extensive discussion of this obviously belongs on "Life and Times", but according to Pool's _What Jane Austen Ate and Charles Dickens Knew_, "it appears that 8% of the children lost both parents by the time they were 15, and almost a third had lost at least one."