With the optional objectives complete, it is time to hunt down Marie Levesque. She is found in the main hall of her house. From where you found her husband, head into the main hallway and deal with the remaining guard. Look to the North. There is likely a second Brute there. Take him down with a run up assassination or a shot from the Phantom Blade. Go to the next locked door in the hallway. Use Eagle Vision to ensure the guard on the other side is looking or walking away before you break through here.
Turn to the left once inside the next room. There is one Sniper watching over the ballroom where you will find your target. For now, take him out. After that, take cover and use the camera to look at the ballroom below you.
You need to wait a while. Your target has high observation skills so it is very important to stay in stealth cover to avoid being spotted and triggering an alarm. When your target gets below you, hop onto the rail and quickly go in for an aerial assassination. Take a little time to aim for your target and ensure you have Marie targeted instead of either of her guards.