I guess I am the only one who is slightly skeptical. If your parents truly have a handle on this, why is your sister borrowing $20,000/yr?
Her stats were good. If her UW GPA was a 3.5 with a 1500, that is far from quantifying her situation as
The difference between a 36 and 1500 is approx the difference between a 36 and a 34. There is a difference, but not enough to leap to the automatic assumption that the outcomes will be significantly different. Once you cross a certain threshold, differences have less significant of an impact. Think of scores being evaluated more in terms of ranges vs this score = this outcome.
For competitive schools and competitive merit, admissions and awards are going to be holistic and extremely competitive. If you have the resume of outside accomplishments to match your test scores and coursework that represents something truly significantly different than your sister, then your parents might have good insight to your possible outcomes. But, test scores and GPA combos by themselves only provide givens for automatic merit.
Best wishes during your application season.
Very wise position!!