Hello,
My wife recently became a US citizen. So we filed I-130's for her parents to come to the USA, but her 18 year old sister still lives with the parents. How will she be able to come to the USA, assuming the parents could come soon?
She can't move to the US anytime soon, unless she finds another way independent of your wife. The sibling category is backlogged 12 years.
You are wrong. The sister qualifies as a derivative beneficiary of the mother.