Vanishing/I's

Main character lives peacefully with his sister after losing both parents. But one day his beloved sister dies as well. The autopsy shows that his sister bore a child. To avenge the humiliation and death of his sister, the protagonist infiltrates her education facility. He chooses the means to insult the officials who could not notice his sister's despair and that way approach the real culprit.

[Edited from vndbreview]