**Spoiler Free Solution**
Early during the first chapter, you'll end up in a bar with several characters. The one you're interested in is Ottilie, the well endowed Asposian a behind the bar. Keep talking to her, going back to her after speaking with others if necessary, and you'll gradually get her to reveal and confess two sins she's guilty of. Make sure you exhaust all the dialogue here.
This will unlock the achievement!
"You have to speak to the Sin-o-Mat before the dialogue options to discuss the Sin-o-Mat and Ottilie's sins becomes available.
Posted by EastX"
The Sin-O-Mat is on the screen down the street outside the bar, behind the Asposian carrying her baby. It's to the top right of that area. Then, return to the bar.