Patapata Hikousen no Bouken

Jane's mother dies when she is born, and her father, a rich English aristocrat, soon remarries to a woman with a son, William, who despises his new father and brother. George and Jane grow up with a dream to make a flying machine. George believes the distant Asian sands hold a secret: a mysterious cerulean sand which can make machines fly. He goes on an expedition to find it, and soon is reported executed for treason. William disappears too. After she recieves an unsigned letter holding a handfu (myanimelist.net)