When I say feminist I don't mean a coffee shop feminist who believes in a generic idea of equality, I mean a tr00 feminist who believes in all the main points of contemporary feminism.
I suspect that not having male siblings makes you kind of oblivious to the male experience in general therefore incapable of really empathize with men and therefore prone to see them as a privileged class undeserving the same empathy and understanding reserved to women. Many of the most belligerent and rabid feminist journalists like Amanda Marcotte, Meghan Murphy, Clementine Ford, Jessica Valenti, Katherine Spillar, Lindsey West or Laurie Penny don't have brothers as I know of.