How much weight does being on a Varsity sport carry over a JV one?

My school's girls' tennis team is pretty competitive when it comes to making Varsity. The Varsity team's pretty small - only about 12 girls - compared to the 45-strong 2-team JV. I have been at the top of the better JV team for the last 2 years (sopre and freshman), and really want to make Varsity as a junior (this year in the spring). The thing is, there are only 4 open spaces on the small Varsity team, and I'm worried that, even if I really practice the next couple of months, others will beat me out. I want to know how this would look to college admissions offices. Even though I do have other extra-currics, I'm worries that I won't earn a varsity letter for tennis by the fall of my senior year when it's time to apply to colleges (I'm applying to relatively elite schools, such as Northwestern, UChicago, UMich, Notre Dame, and Carnegie Mellon). Any insights/advice? Your help is much appreciated! :)