Crime documentary about siblings accusing their dad of murder?

Not sure if this is the place to post this but I have been searching for this for years.

When I was young, maybe 15 or 20 years ago, I watched a documentary about two siblings who claimed their dad was an uncaught serial killer. The documentary was made by a British team (I think), but the subjects were either from the USA or Canada.

All I remember was that the siblings were middle-aged and white, and claimed they had actually witnessed their dad killing people, including soldiers and hitchhikers he’d picked up.

The documentary also interviewed the dad himself, who seemed like a normal, elderly guy who was bemused by the accusations. I think the insinuation was that maybe the siblings were a bit weird and had made it all up.

Any idea about the documentary or the case it might have been covering is greatly appreciated.

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