What do you mean, "she has a plan not to die"? Six months later is when she goes home, and she's not dragging that villainess' dead body around with her forever (she's going home to her own body), so calling it "a plan to survive" is highly inaccurate.
That said, I think she did explain everything to the younger brother (poison/dead body animated by the devil/six months time limit). It was fairly necessary information at the time, and it kept her from being buried/cremated when she was comatose.
The brother knows his real sister died, and that she's carrying out his sister's dream, but he doesn't know she'll die at the end of the six months. She didn't have the heart to tell him that on top of the news that his real sister was gone already.
So the brother knows the truth right? About the demon that cursed her? Or did he only know that she would die soon? I lowkey forgot.
I was just thinkin how the ML would react if he found out she would “die” in 6 months (even though she has a plan to not do so). (But the 6 month thing would be explained so i thought his reaction would be worth seeing if he found out she would die) and was thinking the brother could spoil the truth to the ML but i forgot what he actually knew.. and IF he could spoil it