[7Th]

The anime/manga Arion seems to be one of the primary sources of people in 2ch's speculation thread expecting Kouga and Aria to be siblings:

Spoiler for Arion spoilers:

Arion is the story of a young man that was trained to be a warrior for reasons he doesn't know; in his adventures, he meets a young girl called Lesfeena (who serves as Athena's maid) with whom he feels a deep connection. It turns out they are the children of Poseidon and twins. The connection between Arion and Lesfeena is very similar to the connection between Kouga and Aria and Lesfeena's eye design is really similar to Aria's.