Brother-in-law's exclusive favorite

The younger sister will always do everything according to the older sister's request, from work to men and sex life, everything is under the control of the older sister. The older sister is married and wants to start a family, but she is unable to have children. In order to solve the problem of her troubled sister, she ordered her younger sister to be a surrogate mother for her. Nian's sister never refused her sister's request and eventually had a relationship with her brother-in-law...