User blog comment:FrankieX98/The future members of the Issei's harem and peerage./@comment-31137559-20180916054952

Grayfia will start giving Issei 'private' lessons so he doesn't embarrass himself when he has sex with Rias for the first time. And so as his older sister, she instructs him in the ways to please a woman by offering herself to Issei. And then she wipes Issei's memory of it ever happening. But she finds out she is pregnant with Issei's child.