hi! Thank you for enjoying the game. I’ll go into the spoilers below the cut!
After killing Mother, you should be able to pull out the scissors in her chest. There’s only one tile you can interact with to do this, if I remember correctly you need to do it from the top. Afterwards a white door should appear near the top of the room (takes a few seconds for it to show up) and you should be railroaded into the ending from there. If you still aren’t able to trigger it, please let me know!