In this one she seemed convinced her aunt would die, believed that Edward would give her a share of the money, and willingly shagged him?
As a plot point, it's ridiculous. Firstly, there would surely be another copy of the will lodged with solicitors. Secondly, even if there wasn't, once Clara publicised the existence of the Sanditon will, she would be doing herself out of an inheritance.
Even if she thought it was worth it to disinherit Edward, it's only ever going to be her word against Edward's - where is his incentive/motive to keep his promise of giving her a share, once his aunt has died?