Just to point out your incorrect on one thing. Caroline the older sister, in the beginning she absolutely does NOT want to fuck the MC. A big part of her first three relationships involve her coming to terms with her feelings for her brother and shes currently 4/5 and has not had full sex with him yet. In reference to what
@Vasasasa was saying he's mostly correct, but I'd argue that Sara not Caroline is the stereotypical one for these kind of games. Sara is very much the loving younger sister infatuated with her big bro. Caroline on the other hand is an actual caring older sister who isn't bitchy and convinced her younger brother is a sex offender, which is the norm in so many of these games.
I do agree that a big part of Milfy City's appeal comes from the way it overturns the majority of tropes in these games. Take the mother character for instance, typically they'll be all "but we can't!" until be it by blackmail, magic, drugs or whatever the MC finally fucks her then the narrative tends to peter out. This is because its often the case that sex is the "goal" and once its achieved that's pretty much it. With Linda, Milfy City skips all that "but we can't!" nonsense and jumps right in to her lusting for her son. That's refreshingly different from the norm. In this game sex (and i'd point out that the MC hasn't fucked any of them dick in pussy yet) isn't the end goal, its just a point along the way.
I'd also note that it's only a "harem" if YOU choose to pursue that route. Every girl is 100% optional, you can completely ignore a girl if you want to and it in no way detracts from the other girls stories. Finally the game is only in 0.4, there's still half a dozen none related girls and their stories to explore and once the girls individual stories are completed there is a world of potential for intersection.