We found 2 Reddit comments about The Last Treasure. Here are the top ones, ranked by their Reddit score.
The Last Treasure? not siblings though - a boy and his female cousin
This one is a bit past your read date, but it seems similar: https://www.amazon.com/Last-Treasure-Janet-Anderson/dp/0142402176
The Last Treasure?
not siblings though - a boy and his female cousin
This one is a bit past your read date, but it seems similar:
https://www.amazon.com/Last-Treasure-Janet-Anderson/dp/0142402176