The idea that trans-ness runs in families never quite goes away. All permutations actually happen. One trans person with otherwise cis-straight siblings; a trans person with a gay/lesbian sibling; a trans person with a trans parent; two or more trans siblings in one family.
On-line I find a claim of 39% concordance for trans-ness among monozygotic (identical) twins and zero concordance among dizygotic (non-identical) twins and non twins. Some take the 39% to indicate genetic determinism - but contrarywise the other 61% seems to indicate that something else - epigenetics, different parenting, free-will, self fashioning - trumps the genetics.
Not until the 21st century were there studies surveying a few hundred trans persons with their siblings (twins or non-twins). Even so one can assert that there are enough trans persons that all permutations will occur.
It is an historical artifact that in the 1970s, doctors were writing up single-family examples for professional journals and the journals accepted them without saying anything about control groups, statistical significance or such.
In two of the cases below the authors accept Robert Stoller’s paradigm that transsexuality happens to beautiful baby boys, where the mother (previously a tomboy) extends physical contact by several years, and the father is mainly absent. They then shape their single case study to fit the theory.
A surgeon and a psychologist from Hammersmith Hospital, London and a psychiatrist from the University of Manchester reported on two Chinese siblings, 25 and 23, the 4th and 5th of seven children raised in Singapore. The two, unlike their other siblings, were raised for eight years by the maternal grandmother. The elder worked as a chemist until migrating to England - she then socially transitioned and worked as a hairdresser, model and dancer. The younger after social transition in England worked as a hotel receptionist, hairdresser, and manicurist. Both sisters had somehow had penectomies and bilateral orchidectomies, and estrogen therapy before coming to Hammersmith hospital where they were examined and accepted for treatment. Vaginoplasties were done. Both were pleased with the results.
B D Hore, R V Nicolle & J S Calman. “Male Transsexualism: Two Cases in a Single Family”. Archives of Sexual Behavior, 2, 4, 1973.
Stoller and Baker, both psychiatrists, reported on two trans women, 19 and 17, in the same Spanish-American family. They had already had estrogen therapy. As per the Stoller paradigm, both were regarded as beautiful babies, the mother continued a close physical relationship as they grew up, and the father, in the military, was usually away elsewhere. The elder, referred to as ‘N’ was living with a boyfriend of two years. The younger, referred to as ‘T’ was already employed as a woman.
The authors hypothesize an etiology: “At any rate, in no other family seen in our program in which there is a very feminine boy has there been more than one son beautiful at birth; we wonder then if it is the beauty which sets off the process with parents in whom this potential exists. In all of those families with more than one son, only one-the beautiful one-was reported as very close to his mother, and that one always was the feminine one. This present family, then, conforms to the hypothesis.”
It is not stated whether the two sisters were able to obtain completion surgery.
Robert Stoller & Howard Baker. “Two Male Transsexuals in One Family”. Archives of Sexual Behavior, 2,4, 1973.
In 1974 three doctors and a social worker at the William S. Hall Psychiatric Institute, Columbia, South Carolina, wrote up an account for a psychiatry journal of three siblings, spaced five years apart who each transitioned to female. The four authors contrasted how the mother, with two absent husbands in a row, had taught them to cook, clean house, do laundry, market, and otherwise function in the stereotypical housewife’s role, but then when she discovered that the two elder children - having become adults - were living full time as female and had boyfriends, she strongly disapproved as this was ‘an act against God’. The four authors regard the family as fitting Stoller’s model in that the mother had dressed male and worked in the fields of the family farm until an early marriage at age 14. While the two elder sisters were examined and interviewed they were not offered the desired surgeries. The youngest was unavailable for interview because of parental objections.
Dr Biber mentioned in interviews that he did the operation for three ‘brothers’ from Georgia. This could be the same three.
Robert Sabalis, Allen Frances, Susan Appenzeller & Willie Mosely. “The Three Sisters: Transsexual Male Siblings”. American Journal of Psychiatry, 131, 2, August 1974.