Among other cohabitation games, this could have been average, but there are two distinct flaws that I think plunges this to the realm of being "subpar." Those flaws are the heroine and the sex gameplay.
In other cohabitation games with a little sister, the character of this little sister tends to rely on tropes to some extent-- the bratty younger sister who teases her older brother, the sickly sister who needs to be cared for, etc. The heroine here is just... boring.
The heroine is just idealistic to the point of dullness. Obviously fantasy is always a key part of these games, but this is a little sister whose hobby is cooking for you and who is obviously horny from the get-go. No matter what game you play with her or what book you read with her, she'll like it all the same, she has no preferences of her own. Whenever you travel to an empty room without her in it, there's nothing to do-- you just keep looking around the house until you find her and talk to her basically. At least other cohabitation heroines have preferences for what type of game or book they like, or they're occasionally needy in an endearing way. The heroine here is just a sex doll meant for feeding you and having sex with you.
The sex gameplay is also lacking. For one thing, you need to have sex in order to gain skill points which are necessary for advancing the game. However, having sex in order to gain those skill points takes forever. You're looking at 5 minutes minimum in order to gain enough skill points to advance further on the skill tree, and there are up to a dozen nodes you have to reach on the skill tree. Additionally, if you want to calm down the heroine's attentiveness during sex in order for her to stay asleep, you have to pet her head by swinging your mouse all over your mousepad like an epileptic. It gets legitimately tiring, and there's no reason for a point and click sex game to have such a tiring mechanic.
This game can get the job done if you're hungry for this type of game, but I can only see that applying for people who have already played through the far superior other cohabitation games. If you haven't already played through the cohabitation classics, I would recommend starting there instead of playing this.