plain old parent suck up bitch
me: *does absolutely nothing*

Younger sister: *starts fake crying* Ahhhh mom dad she punched me!

by Maddgymstar October 12, 2019
A beautiful and cute, she can deceive anyone and everyone of her true motives
Oh that's one heck of a younger sister
by BOOM , burn March 13, 2017
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
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
When the older/oldest sister of a family oftens engages in a relationship out of spite of her already engaged or further advanced younger sister. Generally moves in with her new man, gets engaged, married or pregnant all within the span of the younger's term of engagement. In the long run, not a wise decision.

Most people have witnessed at least two of these instances. General outcomes of "YSS" include but are not limited too:
1. a jailed husband (physical abuse, drugs, theft, video taping women at work, etc..)
2. A miserable 6 month marriage
3. single motherhood
4. self pity
Observer #1: "Jamie moved into her man's house, got pregnant and has quit her high paying job all since her younger sister LeAnn got engaged to her her long time boyfriend."

Observer #2: "yeah, I heard her man has a ton of great qualities like...well, umm...nevermind. At least he drives a nice truck"

Observer #1: " Another victim of the Younger Sister Syndrome (YSS)"

by it's funny...unless it's you October 3, 2005
When you have a younger sister/sibling that is better looking than you
Stacie is okay but have you seen her little sister Jessica? Stacie totally suffers from Y.C.S.S. (younger cuter sister/sibling syndrome)
by Discobeck August 28, 2018