Some of the strongest Bleach siblings happen to be Soul Reapers, and most of them are Lieutenant-level or higher.

It seems that Bleach author Tite Kubo intentionally ... Regardless of who the strongest Espada was, each Arrancar in this group embodied a certain aspect of human mortality, their abilities ...

Tite Kubo delved into the Arrancars’ relationship with Aizen quite a bit in Bleach, but it still doesn't make sense for them ...