My condolences, Nanette, for the loss of your sister. See your doctor and inform the circumstances of your sister's passing.
There may have been a rupture of a vessel from soft plaque (plaque between the layers of the vessels) and not seen with tests performed. An CT scan angioplasty could have possibly seen the build up of plaque or a cath with ultrasound. Nevertheless, you should have a heart healthy diet, exercise, don't smoke, etc. Take care.
Opps meant Chest............not Cheat