The Sociophobic Sister I Picked up Became a Yandere by Me

[Single heroine] + [Yandere] + [Two-way redemption] + [Development] + [Good ending] Xu Yan: "Be my sister and I will support you." "Really? You're not allowed to go back on your word?" The girl said softly. ... One year later The girl asked Xu Yan: "Is it really not possible, brother..." Xu Yan: "No! My sister must listen to her brother." ... A few years later "Xiao Xi, calm down, I'm your brother!!!" Xu Yan looked horrified... "Well, okay - then I regret it, I won't be your sister..."