I found an interesting article in the NY Times from a couple of years ago on the admission prospects for twins and triplets when they apply for the same college.

The basic question the article seeks to resolve is "does the candidacy of one sibling of the same class negatively impact the candidacy of another"?

It has always been my perception that selective schools seek geographic and demographic diversity.  For obvious reasons twins and triplets will be the least diverse prospects imaginable, at least on paper.  In fact, my kids have very different personalities, and that is consistent with other multiples that I know.  Nonetheless, environment plays a significant role in shaping one's views, and I think to some degree the college admissions process is a statistical crap shoot, so assuming a lack of diversity seems pretty rational.  If the siblings go to a small high school in a small town, and all apply to an Ivy League School, it seems logical that the likelihood of all being selected is low, even if their applications are very similar.  I'm not sure the article dispels that theory, although Harvard's Dean of Admissions suggests the diversity quota is a myth.  Interestingly, Duke University considers twins individually and as a unit.  I'm not sure how the average twin or triplet would view being evaluated for college admission as a package with a sister or brother, but at least nobody would be ruled out strictly by virtue of one being admitted.