My sister Arisu Sakayanagi is a yandere

I was reincarnated into COTE and my sister is none other than Arisu Sakayanagi. Unlike other normal siblings, we have already done plenty of sex before we even go to Advanced Nurturing High School, the school where canon events take place. My sister is sweet, smart, and flirtatiously cute, but when I introduced another girl into my life whom I saved from a bullying incident, she changed. And it was also the time when hell began to break loose.