Me and boyfriend was arguing?

me and my boyfriend was fighting yesterday and his sister jumped in it is that right for her todo tht and she pulled my hair and that im like 19 and shes like 12 or 13 is it wrong for me to defend myslef or shouls she stayed out of it like her brother told her tooo she also have a very smart mouth so she always get beat up so i dont think thats something new so now my boyfriend mom mad about it