Is Anima a Final Aeon? in Final Fantasy X when Yuna's party is going though the old Blitzball Stadium in Zanarkand they see a memory of a mother hinting for her child to choose her to become the Final Aeon. then we see a young Seymour crying saying he doesn't want her to.

However in the Baaj Temple (where Tidas first awoke in Spira and met Rikku) is a Fayth which is revealed to be Seymour's mother who choose to become the Aeon Anima.

So is Anima a Final Aeon? If so why didn't Seymour fight Sin? if not then what happened in Zanarkand that prevented her becoming the Final Aeon but instead a regular Aeon?
Anima was indeed the final aeon. As for why Seymour didn't use it, well, Seymour started the pilgrimage when he was still very young with his mother and he was not aware of the final aeon and the final summoning back then. When his mother finally revealed her plan to become the final aeon so for the sake of Seymour, it left him devastated. He broke off the pilgrimage there and didn't proceed any further and thus didn't fight sin. This can be verified from the wiki as well.