I couldn’t pass up on a Free! doujinshi that (1) was not yaoi and (2) contained imoutocest and defloration. I also really like Gou. Hope you enjoy it.
QC: Crystalium, Palaxius
(C86) [Blue Topaz (Kuraki Hiro)] Gou GO! My Sister (Free!) [English] [Team Koinaka]
(C86) [ぶるぅとぱぁず (椋木尋)] 江GO! My Sister (Free!) [英訳]