One of our female cats had a spontaneous abortion, and I was wondering if it could have had anything to do with the fact that her brother (from the same litter) was the one that got my cat pregnant?
Inbreeding in cats could certainly result in spontaneous abortions. Inbreeding can result in deformed fetuses, and that, of course, could cause an early termination of a cat's pregnancy. However, many other things can contribute to spontaneous abortions in cats, and there is no way to know for certain whether or not this was the cause of your cat's miscarriage.
Thank you for writing in, and my deepest condolences for your loss.