Edit: RB beat me to it. Read his!
I think it was to show that Sun has lied to get herself out of trouble in the past, and others have faced the consequences of those lies because her lies were not very convincing. Sun's honesty, particularly to Jin, was is great question during this episoode (and for good reason). I think setting it up by showing her lying to avoid getting in trouble herself set the tone for watching her deceive Jin the first time by cheating and the second time by plotting with Sayid.