It's very simple. We always thought baby Ao Yue was another name for baby AoAo and that AoAo was Ao Gu's real brother, when it turns out AoAo is Ao Yue's reincarnation and is technically just adopted, because he's what was left of Ao Gu's real brother Ao Yue after he grew up, died, and was resurrected a-la-Voldemort-style.
Oh, and Ao Yue just took Xuan Yue as his new name, but he's still exactly the same guy body and soul (whereas Ao Ao = same soul as Ao/Xuan yue's, but a different body).
It makes so much sense now! I was really confused. Thank you so much!
I just had 3097 seizures just reading this manhua, someone explain