don't remember if this was adapted into a manhwa or not but the novel was something like this: the MC's sister is the heroine of the horror novel or game. however, the MC is the one who has power to change the story because she has knowledge of the plot and tries to protect the sister from the ML who is a killer.

bump

If you still don't know the answer, you might try asking on the subreddit /r/otomeisekai, the people there are pretty good at identifying these things