ATMachine, I think the “stumbling block” for a lot of people on the “secret Skywalker” thing for Vader’s former identity pre-Vader, is the forgone conclusion that Anakin Skywalker was envisioned as an only child back when Star Wars was made, just like he is in the PT.
This is a good suggestion. That’s an assumption on their part, though, and not necessarily warranted. I mean, you’d think Uncle Owen would be reason enough to doubt this received wisdom.
Even in the 1974 rough draft, protagonist Annikin Starkiller (who was a template for Anakin when Lucas made ESB and ROTJ) had a younger brother, and Luke had an older brother of his own in the January '75 second draft. The idea of a Skywaker sibling predates Vader being Anakin by several years.