I dunno. Basically from my understanding, the whole reason for the deaths and craziness is because Taiyo's dad, Yuuji, couldn't keep his pants on with Ruriko, Saki, and Machiko (attempted murder, suicide, performed human sacrifice).
So I guess the real mystery is why the men of the Nakahara family become such sex magnets (since Yuuji seems to be doing the same with the Miyara sisters and Mar). If you think about it, this could have been another plot for DNA² . lol.