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1.
A theory that states that all younger sisters are either ugly, retarded or mind-numbingly annoying, no matter what. Sometimes, they can be a combination of these, or all three at once. These attributes are more pronounced the greater the age difference between the younger siter and her older sibling.

If the older sibling dies, the younger sister becomes neutral until such time that a younger brother is born, in which case the Older Sister Theory will take effect.

Similar to the Older Sister Theory, if there is a not-annoying, -retarded or -ugly younger sister, then one or more of the following must be true:

1. She is not blood related
2. She has a disease
3. She has addiction problems
4. She does not really have older siblings
5. You are living a lie
Example:
Guy1 "So your sister's a total twat, huh?"
Guy2 "Why would you say that?"
Guy1 "Younger Sister Theory."
Guy3 "Bitch must die!"
Guy2 "Actually, she's not that bad."
Guy1 "Dude! Seriously?!"
Guy2 "We aren't actually related, chill out."
Guy3 "Sweet! You into voyeurism?"
by Scripul April 14, 2010