The Peshawar lancers
S. M. Stirling2002

In 1878, a deadly asteroid shower decimates the population of the Northern Hemisphere and forces the relocation of the British Empire to its southern colonies in India, Australia, and South Africa. Two centuries later, when the British Raj faces deadly threats from rival empires, the crown prince places his trust and the fate of the empire in the hands of a young officer in the Peshawar Lancers and his twin sister, a brilliant and innovative scientist.