Bro is a narcissistic bastard who sees himself in his sister and is therefore obsessed with her. Sis notices his obsession and tries to escape but bro traps her. Sis kills herself and leaves a last letter questioning her bro which we read in the manga, also sis had left a trap for bro to be caught for her kidnapping. In the end the flower reference is about the flower narcissus.
Also after sis dies Bro dresses like her with wigs and everything because he is narcissistic.
OMG THANKS CUZ I WAS CONFUSED
You are welcome :)
Didn't understand:/