Wellllllll, we don't know how he was when he was 15 years old, I mean maybe when he was 15, he wasn't with his sister or she was like gone for some time, so if that's true he already knows where she was, I mean when he was 15,not actually where she is now
When he's 15, it should be around the incident in Ch.4. They don’t have parents, Mongryong doesn’t has anything else besides his sister. That’s how he committed the crime to revenge for his sis then became close to Prosecutor.
I guess he imitates the Alzhemimer-grandma in Ch.1. ٩(๑❛ᴗ❛๑)۶
Ahahahahah yeah
IDK But if he has memories up to 15 years old, then he should have reacted to those scary Yakuza and asked for his sister instead of claiming to be lover of Yakuza boss.