I belive that you only capitalise these things when you are using them as a title, that is "Uncle Jack said something", but "my uncle, Jack, said something".
However, what about when it's used as an adjective? That is "my uncle Jack said" which reads very differently than "my Uncle Jack said". The first has "my uncle" modifying Jack, which is the intended nuance, while the second has my modifying "Uncle Jack", which is to say "my Uncle Jack (as distinct from anyone else's Uncle Jack)".
I think there's actually a grammatical error in the "my uncle Jack said" form; the common usage should have the two commas ("my uncle, Jack, said") but putting in the pauses during speach has dropped out of favour because of the frequency of use, so the written form doesn't match what we hear, so looks wrong when we see it written out.
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