Play through the game normally until you get back to your apartment and are presented with this choice:


Once you get here, don't select either of the options. In fact, don't touch anything. Take your hands off the mouse and keyboard.

After about ten seconds, the screen will fade out and you'll get a little quote from your sister:


That's about all there is to it! The exact line you get will be random, chosen from a pool of 25 options. After this one line, the game proceeds as normal.

You can also trigger the random dialogue in Day 2, but it's a little harder.

In order to be presented with the "Play Ermy's game"/"Skip it" choice on Day 2, you have to be on Ermy's route, which means choosing the actually-I-don't-like-any-of-the-girls choice whenever you're given the opportunity. Also, you might have to play Ermy's game on Day 1, I dunno.

Once you've gotten to the choice, the procedure is exactly the same. Just sit there and wait for the secret dialogue to appear. On Day 2, the line of dialogue is randomly chosen from a different pool of 25 options, so all you sister-dialogue-completionists out there won't want to miss it.

I hope you enjoyed finding this relatively minor Easter egg. Don't forget to like and subscribe, and ring the little bell!