So, hope i got it right; This guy makes a move on this girl and this girl shows him how she has sex with her brother and tell him to keep it secret, then years later a coworker makes a move on him, asking him to keep it as a secret and that triggers this memory?
P3 There is a female who always ride on the same train when MC is on his way back home.
...
Ookami-san: Alright, I'll show you why.
P4,7 (Phone messages) Sensei, shall we have a pretend play tonight? ... onegaishimasu Oh right, you are an only child. Okay let's do it.
P13-15 Hey onii-chan, someone confessed his love to me earlier today. (p3) How did it go? I rejected him, because I already have onii-chan. Hinako, you don't have to worry about me. You have your own life. Yes, I am doing it. Onii-chan you are the first priority in my life. (Hug) No no no, this isn't normal between siblings. Me and onii-chan are the abnormal ones then.
--- @Spunks It looks more like MC haven't forgot about the girl for one second over the years, rather than memory triggered.
P36 What happened on that day connected us two together, so I will continue to keep the secret. P37 Anyone is fine, please tell me if you know who Ookami-san is.