Sorry, I don't know the exact step-by-step for this scene, but it's basically with one of the three sisters. Remember how she almost gets touched by a stranger in the alley? If you're not emotionally close to them or their mother, but you hang out with them anyway, After scaring away the stranger, you can... you know...
Sorry if I couldn't explain exactly how to unlock this scene, but I think this will put you on the right path.