i don't think it's very wise of sister. the only people who can find out the character is nanako are those who know:
-MC draws lewd manga with wife characters.
-only nanako visits mc everyday
-knows about looks and behaviour of nanako as you can't recognise 3d from 2d Japanese drawings.
so the only people who could've deducted it was nanako were people visiting mc.
hentai authors usually hide identity and she's one of them, and not socially active.
Most probably only the sister has found this out, and only because mc was careless, otherwise never anyone would know.
what's the problem it no one finds out? she has no one to look upto and make a character in their Manga.