I need to put together a proper ramble about the "AU" factor of certain source things (how Aqua + Ienzo as estranged siblings could be justified in the actual canon), but for now I can at least drop this screenshot of the initial "theorycrafting" Sky did from mid-October, and the relevant Wiki excerpts—
Some other thoughts include minor design details (hair colors, Ienzo's hair covering one eye [re: Nept]), both potential siblings haven't had much backstory given to the audience, especially compared to most around them (ie the other Wayfinders for Aqua, and a fair amount of the Org for Ienzo, if we consider Union to be backstory enough for Ven, Lauriam, and Elrena), RG and LoD being theorized as "parts" of Daybreak Town that broke off and remain (thus making a connection between the two of them by "home"worlds), and, as a little bonus, the fact Ansem TW has personal connections to both (if we count Blank Points).
Even disregarding Nept, there's just... A lot here from so many angles. I think, from a theory/AU standpoint, it's still pretty esoteric compared to most thoughts on characters being connected, and frankly, it doesn't matter much to us if people see it themselves or are invested in the idea, it's just a fun little "what if?" riffing off our own fictive stuff!