Tell her she can’t do anything until she’s 18. But it is your personal advice that she decides to wait until she’s 25 when her frontal lobe is fully developed.
This is what used to be known as common sense.
Quick trip to the attorney, change the will. Pronto.
Write her out of the will until she repents.
I agree with the 18 - 25 timetable, but I would also hunt up a skeptical clinical psychiatrist. The reason is that there are still odd hormonal fluctuations taking place at older teenage years. Some of these can unhinge about anyone.
Graves’ disease: hyperactive thyroid gland. Rapid weight gain or loss. Depression & anxiety. High or low blood pressure, body temperature. Strange food consumption like lots of sugary foods.
Some forms of cancer. Could be several places.
Improperly forming internal sex organs or hormone balances.