Well, that's a bummer. This is a stupid and boring "reveal", if I've ever seen one.
For one, it's totally tone deaf. I don't know what genre the author wanted to write, but this just doesn't really fit the silly little rom-com that she ended up creating. This comic just isn't serious enough for her death to really matter. And if it does try to take this one seriously, that's likely going to ruin the rest of the comic.
Then, it's not surprising at all. Her sister being dead is kind of the default expectation. The only reason we had to think that she's still alive, is that even for a silly little story like this it'd be monumentally stupid, for her family to not know that she's dead. Like, she'd have to die in an active warzone, for that to ever happen.
Finally it's a complete waste of a character. She could have been used to build a love triangle with Zhu Ci, or lead them on a chase clue after clue, until they find her, but nah, she's just dead. And not even interesting dead. She wasn't kidnapped and murdered, she left no big resentments behind, just died from overwork, like she's an isekai protagonist in the first chapter.