Wait, since when do the parents know she's a vampire? The daughter might have told them, but a normal response to such a tale would be to treat it like some kind of chuunibyo...
Of course, their family doesn't seem exactly normal either...
At least they're in the right place, there's no laws against sibling relationships in Japan(assuming we're talking about consenting adults). About the only things they can't do(legally) is get married, or have kids (and I'm not too sure about the latter?...).
@TNT261 Get off your high 20th century horse, until relatively recently historically, multi-generational housing was the norm rather than the exception, and still is in parts of the world...