Complex sentence: The Duke's mistress with whom he shared the house in Quebec was Madame Alphonsine-Therese-Bernadine-Julie de Montgenet de St. Laurent, the wife of Baron de Fortisson, a colonel in the French service.
Split into simpler sentences: The Duke's mistress with whom he shared his house in Quebec was Madame Alphonsine-Therese-Bernadine-Julie de Montgenet de St. Laurent, Baroness de Fortisson. Her first husband was the Baron de Fortisson, a colonel in the French service.