Hello, Goodbye

Savior Sibling: a child conceived through selective in vitro fertilization as a potential source of donor organs or cells for an existing brother or sister with a life-threatening medical condition.

or

Evan Buckley was born with a purpose to save his older brother's life, while it was unethical it worked. 14 years later Evan is standing outside of Robert Nash's office asking for his legal help. Daniel's kidneys are failing and Evan is the only match they can find. Tired of being spare parts Evan decides to sue his parents for the rights to his own body. The plan gets complicated when more people get involved. Will Evan get the happy ending he deserves, will it be at the cost of Daniel's life.

(Crossposted on AO3)