Thank you to Reddit u/Thunder9191133 for this post where I originally found how to do this glitch.

It's possible to glitch into grannie's house in Epilogue by skipping the cutscene at the same time that Madeline starts to walk into the house. Here's the most consistent way I found to do it:

1. Go to Epilogue, and climb up the step to the left of Madeline.
2. After she climbed up, hold left and almost immediately pause.

If Madeline walks into the house, you paused too late. Restart chapter and try again.

If you pause the game and see 'Retry', you paused too early. Retry and try again.

3. If you pause the game and see 'Skip Cutscene', you did it! Now you can skip the cutscene, and you'll be in the house (but the cutscene will still be playing).
4. Pause again, and press Retry.

That's it. Have fun in grannie's house!

For some reason you can't grab or walljump on the walls of her house. There's not much to do in there, but it's interesting that it's possible.

Feel free to try it yourself! It will probably take a few tries, but it's not too difficult.