Payback is a........

Alex got sick of getting beat up and picked on by Olivia (previously labeled as the dominant twin) so he decided to take matters into his own hands.....He decided he needed to pay back Olivia when she got in his way.

This is the only time that he has been found beating up Olivia, but I am sure it will continue in the future, especially as they get older and he realizes what Olivia is doing to him.