Tonraq and Senna had fertility problems when they were trying to have a baby. They sought out Katara, the best healer in the world, to help them. This spiritual connection to the Aang through his wife, coupled with the timing of Korra’s birth, is what caused Korra to be born as the new Avatar.
Though Korra didn’t know it her mother was pregnant when she left the south pole. She and her new little sibling will have a bit of a Nani and Lilo relationship but overtime will become the most badass sibling team of all time.
-Anon
Tonraq and Senna—Korra’s parents— met during one of the visits Aang usually took to the Southern Water Tribe. While he was showcasing his skills to the audience, a ice shard accidently got away and was flying towards Senna. Tonraq was able to grab the shard before it could impale her. After apologizing to Senna and shaking Tonraq’s hand, Aang developed a very strong feeling that this would be one of the last times that he would visit the tribe. He did not dwell on it. And the rest is history.
