There can be many explanations as to why siblings are in the same school. One is that they may have come from a home environment that stresses education. It is not uncommon that children from the same family do well academically. Another explanation is that they are legacies themselves, meaning their parents or grandparents have graduated from that college and they are now the beneficiaries of being legacies.
From your personal anecdotes, how many pairs of siblings do you actually know that are admitted to elite schools that you say are "many" ? Five? Ten? Twenty? That's just a small percentage of the actual student body.
I don't think you should spend more time wondering if you have a hook having a sibling there, instead concentrate on getting A's which will be more helpful to you.