My family net income is 85,000-95,000 annually, and our family consists of mother father me and sister(also college student).
Considering my situation, I believe my needs exceed the average financial aid given out from WashU.
So, even though WashU offers a very generous aid, but based on my family’s decision, the aid does not seems to be enough, do I have the right to release from the contract or does WashU holds the right?
Has there been any cases that a student could not legally back out even though they offered aid?
Based on what I’ve heard from a Duke admin, they can’t “legally” bind you if they send you aid and it’s still not enough. You just have to make an attempt to work things out with the school’s financial aid office, and if you still can’t afford it then you’re fine. You don’t have to go to the school.
Yes, you can back out for any financial reason (even if you feel it isn’t enough). However, you should definitely contact your adviser to see if you can work it out first. You never know! First get accepted, review the FA offer and then decide. Lots of decisions and good luck! 
Have you run the net price calculator on the WashU website (and every other college in your list)? It will give you an idea what your FA package will look like now.