I would point out the results of Research, particularly the study done by the Netherland's Brain Institute and the study that was done as a collaboration between UCLA and Researchers in Australia. My dad believes that my being TS is related to the fact I was fed Soy formula as a baby, but I doubt it. When he explained his theory, my sister just rolled her eyes. She has been trying to explain to him for years about me. He still sends me cards that say "Son" even after my sister told him not to. I think that I was born this way and there was a good reason for it. When I told my mother, I sat down and wrote her a letter from the heart. I told her how I felt inside, I gave her examples of times as a child that I expressed myself as a girl. Of course, your sister-in-law may not have known you as a child, but you could tell her about your childhood. My mother remembered a lot of the things I did and once I jogged her memory, it came back to her.You could point out that your brother suffered through some of the same things you did, but he didn't turn out to be TS as you pointed out. I told you my example. I have two brothers and a younger sister. My parents divorced in 1985 and Mom raised us as a single mother. My two brothers went on to become a Captain and a Major in the U.S Army. There is not a trace of Gender Identity problems with them. They are both married with kids and are all "Man". It mattered not one bit that they were raised without our dad around.