Devil's Flower and the Smell of Land

A brother and sister kill the daughter of a wealthy man in an attempt to take her place. The sister manages to sneak into the mansion, but the family living there turns out to be nothing but a bunch of eccentric, psychologically disturbed people. A greedy, utterly despicable father. A hysterical mother obsessed with beauty out of fear of aging. An older sister who shuts herself away, self-conscious about her ugly appearance. A genius younger brother who’s a total smartass. The siblings devise schemes to take over the house, eliminating one person at a time...

(Source: Kaiki Manga no Anagura, translated)