I know this answer will be different for everybody, but I'm an only child.
I asked my friend who has a younger sister (both adolescent), and she says that they ignore each other now. I don't know what they did together when they were little, but I do know that her younger sister was clingy and always wanted to be with her.
Other than ignoring each other, what kind of things do you or did you do with your siblings?
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Life ... that's what leaves the mess. Mad people everywhere.