Young teen has a grandmother who is seen as crazy, she talks to people who aren't there and moves around the house miming different tasks to air.
The teen sees her grandma talking about a birthday party and doing something birthday related but specific. Then a few days later its her birthday and that specific situation happens and she starts to think her grandma isn't crazy.
Soon after, her grandma freaks out and is clinging to the bed post shrieking about water until she passes out and doesn't move. The teen finds out (has a prophetic dream?) that 3 days from now, the dam above the valley her family lives in will break and kill everyone, so her grandma experiences time a few days ahead and 'died' in that flood.
Story ends with the girl leaving her family to save herself because she knows they'd never believe her and she grapples with her 'gift' and thinks she might end up like her grandma.
This was probably read 8-12 years ago.