I hate my niece and nephew
Is there a reason why you hate them?
They keep tattling on me to my parents.
You aren't always going to like all of your family and that's okay. Not all family members will get along and that's normal.
No, it's not. I want them dead.
Disliking your niece and nephew is one thing, but hoping that they die is another. It's wrong to wish death on people, especially your family. I think you should keep that to yourself, and just try to avoid your niece and nephew.