A story about a human brother who is forced to procreate with his giant sister-in-law through one-sided sex

On an island inhabited by a tribe of giants, a young human man lives alone.
He is a small human in the eyes of the Titans, but he is diligent, hardworking, and a good worker.
He is respected by the giants on the island, and he has parents and a sister-in-law who found him and raised him.
He is asked to perform a sex act for his precious sister on her birthday.