Abnaxus states something like April is the Spring Dragon which is reborn every lifetime, like the White Dragon (which is her mother...).
Saga is April. She, And Etta were often leaving the House of All Worlds. Each time they leave the house for "Walking" they spend a lifetime in the worlds of stark and arcadia(lets call them the physical worlds). Each time reborn as the new Spring Dragon(April) and White Kin but as another person in the physical worlds.
It is also stated that April is half dragon half human - which makes so much sense as her parents are Etta(White Dragon) and Magnus(which appear human). I'm not talking about April's "real" parents in the physical world in this specific lifetime as April, but her "Spiritual" parents, the parents of the soul that is being reborn each time as another person in the physical world...
Something happened to the last white kin, we don't know what exactly, but maybe that's why Etta didn't return.
Also, April doesn't know all of this because Saga is really young when she goes "walking" as April. Saga barely remembers the whole thing (you get that from looking at the painting of Crow on the wall). Saga probably didn't know a lot of this when she was April and didn't know the mechanic of being reborn when "walking" and was not aware of what she is at that time. Unlike the White dragon that knows she is reborn, that knows she is April's mother, etc...
tl;dr: Saga is April's spirit which reborn as a physical person each time she leaves the House of All Worlds, for a lifetime, then her spirit is returning to the house, and soon after reborns as someone else when she leaves again. Same goes for Etta, Saga's mom, which is the White dragon's spirit.