Really, this is true whether he's family or not--which is why, with or without the i-patch, he self-describes as a "total fucking loser." He does account for how/why he came to live with them, but I don't recall the explanation (edit: in the spoiler below).
Moreover, in some sense this isn't really MC's world, and by extension this isn't his family. That probably wouldn't be enough to get around any platform rules, but until the start of the game, MC lived in a "normal," magic-free (as far as he knew) world--and as it's set up, he would have known; even the dull know it's out there (and that they're in the minority). In his world, there are no elves, no slavery, no catgirls. Suddenly, he's in a world where all of those are present. Miya, Noelle, Grace, and Chloe are there as before, in the same relation to him (or a former version of him, or the guy he replaced, or whatever), but also know about magic, so they aren't the same Miya, Noelle, Grace, and Chloe.