I think it's pretty much accepted here that Aos is the chosen of Ether and being made by him, his son, since Aos made Alice this would make her a third generation from him, thus grandaugther. She calls Ether like that just how Mc calls either Body father or Memory mother, it seems to be a thing for apostle spawns.
If what you want are some proof we can see a few things pointing to that:
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During Valravn's path he talks about Aos, who is described as "too weak to make sacrifices". It seems Syla and Aos didn't want to fight the way Val did and this made him angry thinking both are too weak to do what is necessary in his eyes.
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When Mc mentions monsters who can talk to Val and describes the fairy he gives a very similar response. Now he could mean different monsters, as in Aos and the fairy not being the same yet having something in common but:
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From the silhouettes we identify most of the chosen, Aglaec, Eishet, Rebis and Val we already saw, this leaves Aos and Indra that we are not sure, so Aos could be the red one in the front or the small blue circle at the left.
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We know how the fairy looks at distance, it seems like a small blue circle.
It could be the hexenrigue i guess, after all it looks like a circle so would fit the silhouette but a lot more things point to the fairy.
Also to think that Alice calls Ether grandfather because she is actually related to him implies a lot of stuff that we don't really know, like one of her parents being a monster for example, there is not much to work there for now.