GG-265 My Big Sister's Real Sex Education

Yui, who has already lost her mother, lives with her father and younger brother, Ma-kun. One day, Ma-kun's friends make fun of him for not knowing anything about sex. Yui, seeing this, decides that she must teach Ma-kun proper sexual knowledge in place of her mother, and takes it upon herself to teach him about sex. However, since then, Ma-kun has become too cocky...