Can't remember the title lol

Its a kinda angsty romance where this illegitimate girl replaces her half sister so the half sister won't get killed when she marries this one guy who the illegitimate girl rescued as a child. Anyways the guy does not recognize the illegitimate girl at first and treats her kinda shitty but recognizes her when she tells him her favorite flower. The half sister also gets blown up at the wedding the illegitimate girl was supposed to take her place in. I know both sisters had red hair and the ML had black hair...