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There are many character types that exist in the World of Darkness that we do not allow as PCs in this story. Specific examples include Mages, Changelings, Mummies, Gypsies, and quite a few others. These types still exist in the world, they'll just never be used specifically in this story.
What this means is that your backgrounds can reference these other types, but they will never actually be brought into play. Therefore, it's reasonable for your Garou to have had contact with a Sidhe in his past,but it would be unacceptable to have him mated with that Sidhe, and expect that Sidhe to ever show up in even a single scene.
Please play accordingly.