Exactly this. Losing touch was not uncommon at all in those times. Sometimes siblings just went their separate ways and not every family is/was close. There is so much that we don't (and possibly will never) know.1948 Australia… 3 years post-WW2. No TV just radio and print newspapers. Adelaide back then was a LONG way from his family ties to Melbourne. My family lived near the Princes Highway that connects the two cities. Even in the 70s it was single paved lane in both directions, Not even a divided road. We are talking at least 2 LONG days to drive Mel-Adl, or for this man, a lengthy train ride. It was quite the effort for our guy to get there in the 40s.
My father was one of 10 kids. Not a close family and many didn’t keep in touch. I also don’t think we ever saw a newspaper in our house even in the 70s. News in the 40s was discussed - for example - in the parking lot after church or at the pub bar. Australian pubs usually had a family dining room attached and were kind of a bar/diner combined. With gossip but without visuals I think it would have been very possible for family to not know this was their sibling or friend.
I also bet it was the outlandish theories being gossiped about and reported on the radio - Is he American? A spy? What a mystery! Why would his siblings have ever thought this could be their everyday family member?