When I worked in the admissions office of a graduate school at George Washington University, the school gave bonus points to applicants from elite universities. BYU was not given any bonus points. Stanford and the Ivies definitely were. I still got in with a fellowship., but that was because of my GRE scores and GPA. Ceteris paribus, a Stanford grad would have gotten in ahead of me.

Don't kid yourself that going to BYU is going to give you the same access to graduate schools as Stanford. If you want to go to Harvard for graduate school, Stanford will make it much easier than BYU.