The Irregular at Magic High School: Mayumi Route

A fan-fiction of The Irregular at Magic High School.
Tatsuya, a man who once could only feel feelings towards his younger sister. When he meets Mayumi for the first time at school, suddenly he feels something strange. Over time, these feelings will develop as the pair have more interactions, whether good or bad, and with that, Tatsuya may just be able to regain his humanity; he may just be able to regain love.

Note that some of the original lore and mechanics of magic may be slightly different.

This is the description of The Irregular at Magic High School for further context (Source:Wikipedia)
The series is set in a world with an alternate history, where magic exists and has been polished through modern technology. However, the ability to use magic is determined by genetics, limiting the number of magicians in existence. Following the Third World War, the world's superpowers shifted to these four nations: The United States of North America (USNA), New Soviet Union, the Great Asian Alliance, and Japan. In Japan, the magic community is informally governed by the ten master clans in lieu of the government. Due to the limited number of magicians, they are treated as a commodity and are forced to enter magic related schools and professions. Nine magic high schools exist in Japan; they each specialize in different aspects of magic and are simply referred to by their numbers.
Original Story : Mahouka Koukou No Rettousei
Original Author: Tsutomu Sato
Source: Light Novel