Essentially Aisha, Abel and Abel's Dad are stuck in a reincarnation loop. Aisha killed Abel (and was in turn killed by Abel's Dad) angering God and was cursed to relive the tragedy again and again. The trigger for this is when Aisha regains memories of her past life. The only way to break the loop is for someone with god-like power (Hades) to kill Aisha before she kills Abel.
The reason why the Saintess has not reappeared is because the Temple has been suppressing any Saintesses that appear in the Escliffe line. This is to siphon their power off in order to give to a more deserving person and to try and circumvent the tragedy.
Abel's mother is Aisha in her last reincarnation. She asked Hades to protect Abel and to kill anyone who came near him claiming to have memories of their past life. Once the original Aisha died the soul of Abel's mother took over her body, becoming the Aisha we know.
Aisha eventually regains her powers, discovers why the Temple suppressed them, and begs Hades to kill her.