Physical violence is never okay and in my book, as a victim, there is no excuse for it. You need to get control and take some sort of anger management program or self help scenario. Until you have control over your own emotions I don't believe you are in a position to question her ability to do the same. You must take the higher road. Command respect for yourself by remaining calm.

What is the nature of your relationship with your sister? What I mean is how often do you see one another? Are these visits forced or welcomed? Is bad blood between you two? Are you able to remove yourself or see her on your own terms?