No, the MC met the mother first, probably a few weeks before meeting D. Remember, the story starts when D arrives home from a vacation, at which point MC had already met and been attracted to her mother. It's a coincidence that MC and mother only start dating for real at that point.Honestly, I'm surprised there isn't more of the DMD-style "Can we fuck the daughter yet?!" type stuff in this game's thread. I mean, we hecking met her first and started flirting with her first and somehow we've boned the mom a LOT but haven't done all that much with the daughter by comparison.
The point is, MC is 'officially' in an early relationship with M, not with D.