Sister of Sorrow is a quest under the Paris DLC of Assassin’s Creed Valhalla.
Your objective is to head to Aquila and assassinate Little Mother.
Here’s a walkthrough of Sister of Sorrow in AC Valhalla.
Before you leave for Aquila, you can go to the nearby tavern and speak to Bastien, a Francia soldier. He’ll tell you more about Little Mother.
Head to Aquila
Next, travel to Aquila which is just south of Evreux.
As you reach the area, you’ll see some guards. Get the key from near the guard.
Find and assassinate Little Mother
After getting the crypt key, go to the front door and unlock the door.
Enter the sewers and clear the enemies along the way.
You’ll eventually find a dagger. Take the dagger and talk to the lady. She’ll think you’re an assistant for the Deliverance ritual.
You will then unlock an assassination event should you choose to use it.
Before you can leave the area, you need a key to unlock the door.
Crawl under a small hole to get the key in the next room.
Finally, go to the ritual and you’ll see Little Mother surrounded by her guards. You have to participate in the ritual if you wish to unlock the event.
Otherwise, you can just battle her face to face, but she’s quite tough.
Leave with Richardis
After killing Little Mother, leave the area with Queen Richardis.
You’ll then stumble upon some rats. To clear the path, destroy the crates blocking the grate first. It’s to allow the rats to leave the room.
Swing your weapon to direct them towards the small grate.
Then, push the shelf to block the grate / hole to stop the rats from coming back.
Next, take a jar of oil and use it to blow open a path.
The quest ends here.