The fights with the Sisters in Resident Evil Village can be tense and frantic. The trick is normally the same, you need to get some sunlight and fresh air into the room to weaken them, allowing you to damage them with your guns.
For the second fight, you are attacked in a dark room within the castle, and at first it may seem like there is no way to save yourself. If you look on the left wall as you enter the room you will notice a bookcase/shelf that you can move. Create enough space between you and your attacker so you can grab the bookshelf and roll it to the right. This will expose a crack in the wall.
Now, at the end of the room, on the right, you will find some pipe bombs on a table. You can set these to a quick item by opening your main inventory. This will also pause the game, so you have plenty of time to do it without being cut to ribbons by the Sister.
Throw the pipebomb at the wall and when it goes off, prepare to hit the Sister with everything you have. Pistol and shotgun blasts should do her in pretty easily from that point on. When the fight is over, grab the Crystal Torso and then reach up and grab the Mounted Animal Skull off the wall.
This will cause the bars that were blocking the stairs to retract, and you can go downstairs. When you get there, open your inventory and inspect the Mounted Animal Skull. Look at the back of it and take out the screw, and then you will be able to place the skull on the same spot where you got the Mask of Pleasure. This will cause that set of bars to retract, and you can head back the main room where the four headless statues await you.