As far as I'm concerned, she deserved getting dumped. She lied to the dude. She blackmailed him. She made him dress up as his brother. She had it coming and I don't feel sorry for her in the least.
she didn't lie in a way it hurt his feeling the opposite actually,she lied so that he can focus on her sister and not be distracted with her and she was even willing on supporting him,she didn't lie when it mattered like supporting him with actual true informations on what her sister liked which proves she was actually supporting him,she was even dissapointed when he gave up fast.
the blackmail thing wasn't serious on her part and she wasn't really gonna do it and was just teasing on her part,same with the dress up,such moments shouldn't get taken seriously (an exemple when tsundere girl hit or beat up the mc in a gag rage),you guys should have read enough manga to differentiate between gag/light moments and serious moments,if it was a serious moment she would have stopped immediately,he was even worried about her at the end.
finally he didn't dump her because all of that,not at all,it's only because he was liking her sister all that time he can't suddenly switch in a flash especially when he knows nothing about her
