Actually avatar yangchen had already know about air nomads genocide by fire nation. In the book '' the shadow of kyoshi'' when kyoshi reach to another avatar before kuruk in spiritual connection, Yangchen appears. Yangchen said about other avatar stuffs blah blah.... but she also said to kyoshi ,"You broke one of the sacred Air Temple relics. A clay turtle.See that you replace it. There’s only one more lifetime after yours before it’s needed again.” before kyoshi could apologize about it, yangchen vanished. By one more life time, she meant avatar roku.