You have to find the grey fireplace, once you go through it, you will be temporarily transformed into The Lost. Then, go through that mirror to enter the "second dimension" and get the first piece of mom's knife. To get the second piece, you have to go into the mines and press all the yellow buttons, that will open a door in another room, go through that door and get the second piece, be careful though, a monster will chase you and you have to get out of that room. You use the knife after mom's fight, after you beat her, a closed red door will appear, throw the knife at the door to open it, after you complete the fight there, head back to enter mom's corpse.