Complex sentence: Louise of Hesse was a daughter of ancient German princely family, the Landgraves of Hesse, but lived in Denmark from the age of three and had Danish ancestry, which made her Danish for all practical purposes.
Split into simpler sentences: Louise of Hesse was a daughter of ancient German princely family, the Landgraves of Hesse. Luise was from a German family, but lived in Denmark from the age of three and had Danish ancestry, which made her Danish for all practical purposes.