Memory by Margaret Mahy
When two worlds collide, life begins to make a different sort of sense...
On the fifth anniversary of his sister's death, nineteen-year-old Jonny Dart is still troubled by guilt and an imperfect memory of the accident that took her life. He goes searching for the only other witness to the fatal event, his sister's best friend. But instead of finding the answers he's looking for, he finds Sophie - an old woman with Alzheirmer's.
Sophie lives with several cats in genteel but dreadful squalor, and has no memory of recent events. As a temporary outcast and labelled 'loser', Jonny takes refuge with her. In Sophie's house, past and present merge for both of them. Their accidental meeting changes their lives.