There is nothing like the love of siblings. What I love about DC comics is that in every issue they have to make up a reason to have someone be violently attacked. In this case, Supergirl punches Superboy in the face in issue 2 of Supergirl 4th series (from November 2005) solely because he asked her to not bother him on the farm.
The irony is that Supergirl visits Superboy solely to talk to someone her own age who she can relate to. When confronted, she gets all hot and bothered and Supergirl punches Superboy in the face. She does it so hard that she dislodges his jaw. Seeing this, she pops it back into place for him, and all of a sudden everyone is calm. Seems plausible to me.
In Supergirl issue 2, the Teen Titans make an appearance after the two of them fight (because teens are mature enough to fight battles, apparently). I guess Wonder Girl was jealous after seeing two of them together because she flies down and punches her in the face as well. You see, no one in the DC universe decides to investigate a matter before shooting, punching, flying or swinging into a chaotic clobber-fest. Well, maybe Batman investigates. Sometimes.
If you are a fan of the girl of steel, consider picking up Supergirl – The Archives, Volume 1. (affiliate link) It includes her entire original origin, straight out of Action Comics.