I found it. It's a defeat event in the occupied village, you don't need any specific unlocks. Just need to warp over to "occupied village". Walk to the right, choose to "help", then "fight", then just lose the fight. A defeat event will start playing in which the bodonic tribes chief will explain that in their culture sucking milk off the breasts of a strong female makes a young boy become a strong warrior. So he decides to let his two boys suck on Airis and Brans tits, you can choose which girl they pick first, after the scene you'll get a special gallery mode to see the other scene... enjoy I guess.