Actually, Worf did mention in the first season that he had an adopted brother, so Nikolai Rozhenko didn't come completely out of the blue.
I seem to recall a complaint many had about TNG season 7 was for some reason the crew kept reuniting with their family that year, and in many cases learning about unknown family members. The premiere has Data's brother, then there's an episode about Geordi's mother which also features his father and sister. Then Deanna's mother returns and we learn about Deanna's long lost sister with her father also making a cameo. Then Data meets his mother and has a chat with his father. Then Worf's adopted brother. Then Dr. Crusher attends her grandmother's funeral, with granny's reanimated corpse making an appearance by the end of the episode, and granny's ghost lover featured throughout the episode. Then Wesley returns and chats with his dad's ghost. Or something. Then Worf meets a grown up Alexander claiming to be some sort of functionary for the House of Mogh that conveniently Worf had never met previously. Then Picard meets a young man he believes to be his son. Then the Enterprise itself gives birth to a child entity.
TNG season 7 was all about family.