Twins and Siblings
I have always been surprised when twins matriculate at the same college, particularly when they weren't restricted financially and had many choices. I work privately with many twins, however, and they always seem to be very different people and don't wind up at the same college. From what I've read on this topic, there is no policy about accepting twins, but colleges are sensitive to this issue and don't really like the idea of taking one and rejecting another. (I have definitely seen this practice in private schools in my area.) As far as siblings go, there isn't necessarily preferential treatment. As elite schools get more competitive, there are constantly more stories about legacy rejections, even among donors. If that is the case, siblings certainly won't get special treatment.