The plot thickens! Poor Romm had a near death experience just trying to investigate "the thing" hanging around Michiru!
They framed the sister as a possessive stalker so far, but with this series' tendency to bait you into blaming the "obviously" evil characters, and with how Michiru is even more creepily possessive, literally breaking-and-entering, not hesitating to use violence (that bloody pen flashback?), she could be just trying to keep her sister from getting to close to others, knowing how bad things can get if she does.