The relationship between Joseph of Arimathea and Mary is not precisely stated.
Another theory is that Joseph is Marys uncle, great uncle to Jesus and apparently his adopted father.
For the folks that understand the story the father of Jesus is not listed in any official state document.

The situation is that the first century AD account of Jesus was written after the fact, and translated by folks who had a vested interest the statements.

The tin mines in Cornwall open in 2150 BC.
There are ruins of an Abbey built by Joseph.
The fact that Joseph was in Wales/Cornwall - Devon is documented in great detail than the Jesus story itself.
It is also thought during the 30 years not mentioned in the Bible Jesus traveled with Joseph to Cornwall.

It the Cornwall thing a fabrication?

Perhaps! However there is more recent repetition of the details than for the events in the middle east.

For me I was just wondering around Wales/Cornwall-Devon looking for relatives when...