Alex, yes, I'm a bit puzzled by the responses in this thread, too. I've often referred to "my brothers" or "my sisters" when talking about transsexuals as a group.
I suppose it might be a bit different when referring an individual. Years ago in the past, when I think the sister/brother honorific may have been used more frequently, I had one friend who actually expected me to grant her the favors I would reserve only for birth relatives. Perhaps the community has distanced themselves from using the brother/sister honorific as a way of limiting demands on an individual's scarce resources? I dunno.
I think I would still consider an individual, who was also transsexual, as a brother or a sister. But the term wouldn't enter into our conversation, unless he or she was especially dear to me.