They're very competitive. The problem is all premeds (and those interested in PhDs) want to come to NYC and spend a summer there. I've applied to both Sloan and Sinai and got rejected from both of them before. Getting into a summer program is honestly a crapshoot (and I did an AMGEN). I've heard that NYU gets over 1000 applicants for <30 spots or so. From hearsay info and one of the postdocs I've worked with who was in the program 10 years ago, Sinai prefers to accept people who have worked in one of the labs at Sinai (and he said the whole program was pretty much undergrads who worked in labs there). As for Sloan, I have no idea what they look for.
If you're applying to summer programs, I recommend applying to a lot of programs (>10+) if you're dead set on going to one (as long as you're a rising junior or senior, current freshmen have a very small chance of getting in). Be more area friendly, and the less competitive programs tend to be the ones that don't provide any housing or accommodations.