My nephew and niece are constantly fighting. She is 4 and and he is 8.
He annoys her all the time and she responds by lashing out and screaming and crying. Parents (dh's sister and her husband) are generally loving and attentive parents but they do fuck all about the fighting.
My niece told me over Christmas that she wants to chop off her brother's willy and burn it. 
My sense is that my niece feels incredibly frustrated and helpless as her parents just stand by when her older brother annoys her. They never intervene claiming that it's better for siblings to work their issues out themselves.
AIBU to think it is not normal sibling rivalry if one sibling wants to essentially torture the other sibling and destroy their sex organs? The girl is 4 ffs.