Brother pokes sister with her cut-offs down in front of the mirror

It's not clear why, but when my parents are not home, my brother acts so strangely that he molests his own sister. He knows he shouldn't do it, but he wants to have sex with his beloved sister, who, frankly, doesn't resist much. They stand each other up, well, and while they are alone at home, engage in incest, during which the brother fucks his sister in all the tight holes in front of the mirror.