I managed to do that. Iirc, you need to make your parents NOT leave by talking to them, just ignore them. Then after the sister gradually starts to get used to going outside (because you're fucking her closer and closer to the actual "outside"), she finally can leave and you can fuck her outside on the streets. I'm pretty sure there was something to do with the rightmost tree outside, I guess you fuck your sister once more by that tree and the mom comes and thanks you for getting your sister to leave the house, then you can fuck the mother if you want and the game ends... or something like that.
For a very low effort copy of the concept in Summer Memories, I kinda enjoyed that game. Still don't think it's worth the purchase, though. Guess I'll play it again some time.