Overall, this is an okay game, but you have to take it for what it is: a porn game. If you're looking for story elements that aren't strictly driven by sexual desire, this game might not be for you.
It essentially starts off with you wanting to fuck your sister, Mia, and will try all the tricks in the book to do so. Mia, being your older sister, hates you (as in all of these games), but eventually ends up being to horny to resist. Having played Melody, DMD and other similar games where everything revolves around slowly building a relationship with the main female character, I at first had problems with Mia and MC starting to fuck. As Alita below me says, there isn't much in terms of a credible reason for them doing so. However, after you just accept that this is the case, there is a tonne of super hot content and great sexual action, that is well worthwhile playing for. I also think that there is a lack of games centering around creating porn (in the game that is), but Living with Mia does this in a great fashion.
Other than this, I think all the characters look amazing, especially Mia, which also makes the game great to play.
While I, as many others, prefer a Ren'py VN over RPGM, there isn't too much of an issue with this in Living with Mia, as it is obvious and explicit most of the time where you need to go. Sometimes, again as Alita said, you have to run around trying to find things, which is quite annoying but also manageable.
Either way, I recommend trying this game out for it's beautiful characters and great sexual content.