Creating a village and characters is easy. Anyone can do it. It just takes time. To create an interesting game requires much more than that. It requires skill to build a relationship like the one with the sister and it requires a lot skill to keep the scenes with them that take hours to play through interesting. The dev ran away from that like the coward he is and started working on scenes with secondary characters.
This is part of the game, not the whole game, as mentioned before the game title is Runaway Princess, not brother and sister live alone in small village. She is not an adventurer, she stayed behind while he travels but I am pretty sure the dev had mentioned her continuing to be apart of the game from his original plan. He did a great job building their relationship and day to day life at home,h e expanded on the game world and then lost motivation, ideas or time to continue working on it. You could say he was bad at prioritizing or maintaining consistency, not that he was a coward. If anything it was probably to show how far from his comfort zone the main character was going, a little like Bilbo going on an adventure. He is young, he is used to his lazy village life and now he is going out and exploring the world himself, meeting new people, experiencing a new life. If the game continued he could have returned home to her at some point, maybe to bring her along to various hub locations once he established himself in a town or city, but instead, the game was abandoned. Hopefully the remake includes all aspects of the original and builds on them.