I have wondered too how they all know that she is dead. NO one has said how she died or what the circumstances where. I also don't know why she would leave her shares to cliff since she couldn't stand him and she also blamed him for her mother's death in the plane crash. Back then, I think she would have left her shares to Bobby or even to JR to just stick it to Cliff even after her death.
HOWEVER, for the sake of the show, I"m just going to go with maybe before she died she and Cliff reached a level of peace. Maybe JR did her wrong and she hooked up with Cliff to help bring JR down. This all is my understanding that maybe it happened in between the old and new versions.