Kasumi's sis really dropped the anvil there that should really be in every narrative based around relationships by now. Too bad so many authors seem clueless as to how to create good relationship drama without the old "confusing your lover's public face with their private face" trope.
That's why it's so good. It was clearly going down the route of that trope, but then suddenly her sister just steered the entire manga into the proper logical direction/conclusion that one would normally take.