Why is it always my fault?

just about an hour ago, i was talking to my cousin then my sister was just beside then she just hit my head very hard so i hit her back also then she hit back so i also hit back then she hit back again then i hit back again... then she didn't hit back already so i shout "u started it first!". if u all think im wrong, find me! but let me tell u something before u find me. but in the end she say i start it first but did i hit her first?!
The reason why i hit her back: because from small, i had tolerated her abuse already and i just want to let her know that I'm not someone whom she can always beat when she thinks the person is wrong without getting facts right.
Facts about me: I don't go round hitting people for no reason! There's a limit to where I can hold the abuse. I'm quite a brave person and knows how to control my emotion so count her lucky that i didn't use my strength because if i use, think she WILL lose.
Well, to be honest with u, this war isn't over again because I know she'll tell my mum about it and definitely, my mum will take sides on her and its because my sister will tell a different story to make it that she's right.
Even though I'm young but I know that I have a clear concious. So this is my future plans if she tell:

1. back fire and tell the whole truth which is "i was just talking to my (cousin) then sis go and hit me so hard so she started it first lor... i tolerated her abuse already ok?! this time its she started it first. if u think im wrong then fine but let me tell u, this isn't over yet. when i grow up lets see who will be more useful. when she's in trouble dont expect me to help her!
2. name the facts why im right
3. ignore and walk away