Complex sentence: The young prince Frederick, together with his two elder brothers, were with their mother the landgravine Mary and became since 1747 fostered by Protestant relatives and soon moved to Denmark, to be guests of her sister Louise of Great Britain who however died in 1751.
Split into simpler sentences: The young prince Frederick, together with his two elder brothers, were with their mother the Landgravine Mary and became fostered by Protestant relatives in 1747. Soon the family moved to Denmark to be guests of her sister Louise of Great Britain, who died in 1751.