"The couple returned not only with the salvation they sought, but twin daughters—Morgana and Kayle."
"At the mountain’s summit, she was chosen as a divine vessel for the Aspect of Justice, wielding a sword that blazed with a fire brighter than the sun.
Not long after, the twins were born."
I don't know how much time Mihara and Kilam had for boning while climbing Mt. Targon. But, from the nature of Morgana and Kayle's abilities and appearances, I would probably guess they are born from the Aspect of Justice and Mihara was just a physical vessel for these two.