To elaborate, this is actually a Visual-Novel Game. It has a strong storyline, with a bonus-section that is sort-of a free-roaming game. While the story portion also has some game-elements to let you know what your disposition progress is. (Eg, heading south, or crossing borders, or on the main-path.)
There is more involved, but that is the general gist of it. Story -> Bonus-Free-roam -> Bonus-Bonus-ends.
The story is setup around a 34 year-old man who is living with his 23 year-old sister. (May have an option to make her "just a room-mate".)
The story begins with her begging her brother for a real important favor. She is desperate for some physical attention, and found a beautiful woman who is willing, but the woman also wants a man, in a three-way. She too, is sort-of sex-starved.
The brother eventually collapses, but… He has a secret "issue". Not anything to do with banging this random chick, while his sister is around… In his mind, it is no different then sharing the same girl with a brother.
His "issue", is the fact that he, well, has a reference for younger females. (Sort-of why he has been "dry" for so long, avoiding all relationships. But years of solitude rise to the occasion when opportunity comes knocking.)
Ultimately, he agrees, and the arrangement is setup for that coming night. They keep their distance between one-another, which isn't what their guest seemed to have in mind.
This woman, eventually, while having both in the heat of passion, begs them for more, offering "forever" with them, if they "cross that line" for her, for them… Claiming to see it in their eyes, that they want it too.
{There is a point here to continue without doing it, which takes the story into a whole new area. This having a good-end, with a similar bonus-free-roaming, but not the intended ending. Rejecting the sister interaction here, leads to a three-try, later part. Full rejection takes you to the other end. Accepting her lead, takes you to the intended ending, but with a different set of limitations.}
Any-who… You did, or didn't cross that border, and it is now the morning after…
The guest has gone, and it is just you and your sister having breakfast. No weirdness, though weirdness was expected… You still feel obligated to "come out" to your sister, if this is going to continue. Not that you are trying to get out of the situation, but you would rather continue this/anything, with a truthful "all cards on the table", and risk whatever losses or collateral damage comes your way, by telling her. (So you are not dragging something on, which may later hurt both of them.)
But… your coming-out speech gets interrupted… A phone-call from your guest the other night, now talking to your sister.
She tells you the good news… Your guest wants to come back tonight, but she wants to introduce both of you to her daughter…
You cringe at the thought of NOW mentioning what you were just about to mention, but continue anyways. Feeling now, that it is more relevant to say.
You tell your sister what your "complication" was, the other night. She gives you odd looks, but doesn't seem to be upset or disgusted. However, she begs that this not come-out tonight, to their guest… Demanding that she too, must know this, if they are to continue. She tries to call her back, but the conversation is cut-short and their friend has now decided to come-over early, so they can say what needs to be said.
Their friend realizes something is up, but she thinks they are just feeling guilty and trying to back-out, or thinks that maybe trying to bring her daughter over was "rushing the relationship"… The two siblings, however, get the feeling that she is upset and is thinking about ditching dinner altogether.
However, she does show-up, with her daughter, and a little pre-dinner conversation takes place outside. Again, this does not seem to bother their new friend as they thought it might. She insists that they head inside and continue the night. Almost with an encouraging or egging-on underlying sense of urgency. (One both of them think they are just misreading.)
Eventually, the night ends-up with a playful atmosphere, if you do everything as "intended", by the story-line. (Things can get turned around for better or worse, depending on your actions and views.)
The night, which should have ended-up with the daughter being "dropped-off at home", is now ending with a bottle of nearly alcohol-devoid grape-wine, and a twisted game of "spin the bottle", which was originally protested by the sister, but the friend insisted that it was only twisted if you made it twisted. It was an innocent game, if you played it innocently. "Just because you used it to kiss boys, when you were a kid, doesn't make it twisted. That makes you twisted… well… normally twisted."
So the rules get explained, and torture to the protagonist ensues. All in good fun, his games get turned against him, at first.