100 years has passed since the death of Avatar Korra, it was automatically expected that a new Avatar would be born from The Earth Kingdom but nevertheless none was born, both the Red Lotus and the White Lotus still knew that there will be another Avatar but after 100yrs they'd never expected that a Twin Avatar will be born as a new beginning of Avatar Cycle, Korra ended her connection to her past reincarnations thus ending her connection to the new Avatar Twins. But she left documents to White Lotus to lead the new Avatar because she knew that with the New Harmonic Convergence she started a new cycle but just for Raava but also for Vaatu thus Twins born with both connections to Raava & Vaatu are now set. In a ghetto area of Ba Sing Sei a fire nation girl immigrant has now given birth twins alongside his Earth Kingdom Miner husband, upon being born they both noticed that each has a birth mark along the their hands that when they hold hands together would give a symbol of Yin & Yang.
Earth Kingdom Father: Let's give them cute & simple names I'll call this child with the white symbol Whitey & the other one with the black symbol blacky? alright? hon? hahahah
Fire nation girl immigrant: (With a smirk on her face) Yeah that would be great I'll also start to call you Browny? hahahhahha
Yin was the child that was born with a white mark on her left hand while Yang has a black mark on his right hand. Upon growing up you cant tell apart while their personalities do separate them & makes them totally opposite.
YANG: A new Avatar born with a starting element of Earth, a more stoic & reserved character but intelligent & wise. He was born with white symbol of the Diagram of the Supreme Ultimate.
YIN: A new Avatar born with a starting element of Fire, aggressive and cocky attitude but is funny & cannily. He was born with Black symbol of the Diagram of the Supreme Ultimate.
As the twins grew up with resenting each other, they would always argue about a lot of things. So their parents have to separate them from fighting each other, even at a the age of 4 yrs old they were able to master each of their main element, and would often tease each other with Yin starting off by blasting fire balls to Yang, while Yang would usually ignore Yin by looking away & would fight back once he is off the edge. Yang would sometimes fight back by forming a knuckle fist with earth cover and punching his brother away from him. Yin would also fight back by blasting him with fireballs on his face. It was a normal childhood with normal hardworking parents. Not until both of them manage to use another element, Yin was able to get his hands on Air Bending abilities while trying to out run Yang he was able to blast air palms on his hands to fly away. Yang got his water bending abilities while trying to get revenge at Yin by throwing him cold water. It didn't took time until both White Lotus & Red Lotus noticed, bringing them to investigate further.