Skin transplant was done from sister to brother. After few years, brother to sister skin transplant was done, but rejection occurred. This phenomenon is known as

Eichwald-Silmser effect, which is a unilateral sex-linked histoincompatibility. While transplants between same sex siblings are successful, grafts from female to male are also successful, but an exception is seen when the donor is a male and the recipient is a female. Such grafts are rejected as the grafted male tissue (XY) will have antigens determined by the Y chromosome which will be absent in the female (XX) recipient. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition