- brother and sister get caught in a rain storm (anywhere from 8-13 years old...)
- they travel back in time to right before Great Fire of London
- another rain storm brings them back
- no obvious reason (to my memory) of why they went back
- I think they helped people because they remembered reading about the great fire and knew of the devastation.
- I remember it as a great adventure..
Somewhat similar to The Ghosts by Antonia Barber, which was made into the film The Amazing Mr Blunden. If correct, the rain is actually a mist, and it's a house fire, not the GFOL, and (spoiler) you forgot the ghost who is causing their time travel. But the similarities are a young brother and sister sent into the past to save people who died in a fire.