An older gender bender manga about a high school girl who is dating a guy who turns into this raunchy girl during certain circumstances like when the sun is down or something along those lines. He has about 4 or 5 siblings who are also effected by this 'curse' I suppose.

In one chapter for some odd reason the girlfriend gets transformed into a guy. And also once the transformation doesn't revert.

I'm looking through the genre in hopes of finding something but until then I'm hoping someone might remember this one.

Kedamono Damono?

"The oldest and strongest emotion of mankind is fear, and the oldest and strongest kind of fear is fear of the unknown"

That's the one, I was wondering if they ever continued with it. Thank you.