Weaker chance of admit if classmate has already been admitted to graduate program?
I am unaware of any graduate program that makes an active effort to diversify with respect to the undergraduate institutions of their students. If our top 10 applicants were all from UC Berkeley, for example, we'd have no qualms about admitting them in exclusion of applicants from any other school. Thus in a direct causal sense, it shouldn't hurt you at all that another applicant from your school has been accepted (other than the fact that there is now one fewer slots available in their incoming class).

However, knowing that a fellow student has been admitted already does change your conditional probability of being admitted, for the worse. Given that at least some acceptances have been sent out and you are not among them, it is now less likely than it was before that you will be accepted. But this logic applies equally well to any acceptance from any school; it is no more discouraging that the accepted student happens to be from your institution.