兄貴失格!? 妹たちとイケナイコトになってます。

There are two very close siblings. Their free-spirited parents work overseas, but the family has large savings. They live a comfortable life, with no inconveniences. One day, the protagonist's father, who lives overseas, calls to announce that he has adopted a new sister. Because the sister (stepsister) has returned to Japan alone, the protagonist is being given a serious request. The two want to warm up to their unexpected stepsister, but she is traumatized and rejects others... As the older brother, the protagonist struggles every day to become a true family.