It's not like what she ended up doing instead was any better? lmfao. And did she try to talk about it before or after MC comes back to the past? MC's past experiences give her good reason to not trust her.
And I heard she ends up getting redeemed anyway and is a secret genius - but the author has failed to establish this in any way. The fact that she's supposed to be the smarter twin actually makes her behaviour worse. If she was a genius, she would've been able to claim what she's owed regardless of if her sister believes her. Take it by proving she's qualified, not by pretending to be the sister that's so inferior to her. Compete with your sister directly. She wanted to take revenge anyway, so she has literally no reason to pretend to be sick, since she doesn't care about her sister's wellbeing atp. Instead she picked the most pathetic and inefficient way to go about things. It's clear that the author wanted to write a character who is a genius in theory, but had no idea how to actually depict one. Neither smart enough to be a genius nor sympathetic enough to get a pass for it.
the spoilers change nothing, bfr yall. "oh no she took it for granted" so like? talk to her about it then?? don't scheme to rob and murder her, jfc. the mc is who the mc has always been. the spoiler only explains why the sister got twisted, but it in no way justifies it.