What does it mean when you have reoccurring thoughts of killing your sister?!

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Maybe she wronged you. Maybe you don't remember.
How could we possibly help you with this?
You should know best.

It depends. If these thoughts come to you unbidden, you find you have trouble shaking them and they cause you distress then it could be a manifestation of obsessive compulsive disorder. With OCD the fear that you are going to kill someone, or have killed someone, can sometimes occur. However if you are not worried by these thoughts and do not show any other signs of OCD then you probably have a morbid fixation, as do I, and so long as you avoid killing anyone you should be fine.

It's not uncommon to have such a thought - but it is uncommon for it to be 'recurring.'
Try to break it down to the root cause. Basically there are two root causes: your sister, or something else.
1. your sister. the reality is that children compete for limited resources in the family. Maybe this is your response to her getting things that you want, or percieved injustice where she came out on top. This is just plain old sibling rivalry. Believe it or not, it disappears (just like magic!) when you both grow up and leave the family home, and will become close friends.

2. something else. Maybe you're feeling alienated in other parts of your life such as school, or distant from your parent/parents.

Without knowing context, it could be any number of variations on these themes, or something else altogether. I suggest you put some thought into what is going wrong in your life that is causing you to be so disconnected from your own emotions.

Ummmmm
guess if your really mad at her but, when i'm mad at my brother, i don't think about killing him in diffrent ways........

no, it is not, you might want to consider a doctor