Yeah don't think this is going for the poly route, though who knows, it's still early chapters. Still, they just spent the whole day locked in Mizuki's bedroom doing nothing but flirting and talking. Wouldn't it be funny if they never got around to actually doing anything and the sister was completely oblivious until they got together?
Though honestly, I'm here hoping they kill someone and bond over covering up the murder, it's just so romantic <3