There is a reason why Takanari is Shizumasa's "shadow". In the Togu family, siblings were often forced to marry each other in the past in order to keep their bloodline clean, thus having weak and fragile children. According to family tradition, if identical twins are ever born, the current head of the family chooses who will be in the light and who will act as the shadow. If the one in the light becomes ill, his/her shadow will pose as him/her, until a "trial" is performed to determine who will stay in the light.
Takanari and Shizumasa's mother Shouka was also a "shadow" for her sister Kyouka.