My Sister Picked up the Male Lead

My name is Amy. Reincarnating as the little sister of a character from a book I read in my previous life was great, but… On my 16th birthday, my older sister said she had something special prepared and picked up a guy in the forest?!? "Picking up" a "guy" was surprising enough, but that guy was actually the Mad Archduke (…and male lead) who was a child by day and adult by night because of a curse-like spell! He was "Lennox Iberich." "Who are you." "I-I was the one who picked you up and brought you here!" To prevent the original plot line of my sister dying after picking up the male lead, I had no choice but to raise the death flag instead. But… "Amy. I can only see you at night. There's no meaning to a night without you." …Huh? This… wasn't it a "death" flag, not a "love" flag…? ---...