Dutch here probably means no more than that the person described is not a genuine blood relation. In the mid 19th century I will talk to him like a Dutch uncle (meaning…I will give him a lecture) was noted as being an American expression.
1999 - Daily Telegraph - She was the kindest of Dutch uncles, always prepared to listen to one's troubles.
RELATED IDIOMS :
bravery induced by drinking alcohol
The phrase Dutch courage stems from a long-standing British belief that the Dutch are extraordinarily heavy drinkers.
share the cost of something equally
An outing or entertainment paid for in this way is a Dutch treat and sharing the cost of a meal in a restaurant is eating Dutch.
1993 - Vanity Fair - He insists on buying his own tickets, going Dutch, as he puts it.
in trouble - US - informal - dated
1939 - Raymond Chandler - The Big Sleep - And for that amount of money you're willing to get yourself in Dutch with half the law enforcement of this country?
that beats the Dutch
that is extraordinary or startling – US
I am a Dutchman
used to express your disbelief or as a way of underlining an emphatic assertion – British
1994 - Ian Botham - My Autobiography - I read somewhere that Warne said he had been possessed by demons. Well, in that case I'm a Dutchman.