Complex sentence: The young prince Charles, together with his two brothers, were with their mother the landgravine Mary and became since 1747 fostered by Protestant relatives and in 1756 moved to Denmark, to care for Mary's late sister Louise's underage children.
Split into simpler sentences: The young prince Charles, together with his two brothers, were with their mother, the landgravine Mary, and were fostered by Protestant relatives since 1747. In 1756 they moved to Denmark, to care for Mary's late sister's Louise's underage children.