Technically sexism is when you discriminate (or "treat someone unfairly") because of their sex. What is your definition of fair? I can't see an unfairness in refusing to call a man a "Sister"... Because a man in a sisterhood is not equal to a woman in a sisterhood. It's a mismatch of gendered nouns. So a man even if he were admitted would be treated unfairly. A self-identifying male would also not wish to be referred to as a "Dame" or a "Girl" or a "Lady". And to be more nitpicky, we exclude a gender not a sex. Therefore, the real issue lies not with our admission policy but in our name itself. We should change it to "PEOPLEHOOD"! Or not.