<p>I've always found it strange that Aria was said to be a spirit and the mother of Ais, but spirits aren't supposed to be able to birth a child? (unless I missed something somewhere) But I doubt that she was simply adopted as well since there are uncanny similarities between her and her mother. It could be sealing magic, it could be area based time-freeze magic. She also has memories of her father training and talking to her mother, so it obviously can't be reincarnation.
</p><p>The other weird thing related to spirits is that Welf Crozzo became the first in a long line of Crozzos to obtain his Crozzo Blood skill that allows him to make magic swords. That happened on his tenth birthday when he got his falna, since he's said to be 19 this would make it 9 years from the current time, which was around the same time Aiz joined Loki familia at age 7. Not saying there is a connection, but two spirited related things happening around the same time seems like a big coincidence.
</p>