Guardian of the Horizon - Elizabeth Peters

Banned from the Valley of the Kings, Amelia Peabody and her distinguished husband have returned to England with their 19-year-old son Ramses and their foster daughter, Nefret. Ramses is secretly in love with Nefret and plans to flee to Germany to avoid temptation. Then a mysterious visitor changes the plan for the whole family. Set in the Sudan, this is another exciting adventure which follows the Peabody family as they confront all the forces against them armed only with a crumbling map and an important letter...