The story is our MC has always liked the mother and finally gets a chance to manipulate the mother into a relationship. In regards to relationship with the girlfriend you only have control over whether she cheats on you or not. You will always end in a relationship with the mother (can be corruption or love route). If you like the mother then this is absolute peak. Be warned the MC acts like a massive creep in the early chapters regardless if your choices so bear with it because it gets REALLY good midway through the game, if you find creepy MCs REALLY repulsive then I'd recommend skipping through the first few lewd scenes.