Did an uncle or any Darwin invent the steam engine? Absolutely not. Charles Darwin had no relatives that contributed to the invention, nor development of the steam engine. The confusion sometimes centers around James Watt who improved the steam engine, but he has no known relationship with Darwin. In fact, there is no known business relationship either. Meaning, this often-repeated tidbit about Darwin is just wrong. While James Watt, born in 1736, did make improvements to existing steam engines, he had nothing to do with the invention of it and he was not related to Darwin and the Darwins did not collaboration with him.
Thomas Newcomen, born about 1664, is frequently credited with inventing the steam engine in 1712, but his “invention” is more of a “rediscovery.” I say he “rediscovered” it because inventions are frequently “invented” or “discovered” many times before they settle into the Grand Rational Framework of common knowledge. Newcomen’s effort is better described as development of the first commercially successful steam engine. The earliest known inventor of the steam engine is credited to Hero of Alexandria in ancient Greece, born about 10 CE, give or take a few decades. However, Darwin did grow up in an affluent and scientifically inclined household, which is likely why this myth took root.