The legal definition of incest varies across the world, even throughout the United States.
Out of curiosity, I went through Wikipedia's
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, and did a count of the "prohibited relationships" (i.e. what each U.S. state considers incest), and if I counted right,
27 states included adopted/step relationships as incest (54%), 19 states only considers blood relations (38%), the remaining 4 were unclear in that list (and I couldn't be bothered to dig deeper). And out of all of them, Rhode Island is the only U.S. state that only prohibits marriage, but fully allows incestous sexy time between consenting adults (or rather, people 16 years and older).
In essence, most of USA tends to agree with the site's tag rules regarding the definition of incest.
Also, I looked into the legality of incest in the country where I'm at, and those laws considers incest as between step and adopted people. (Again, I only bothered to check my country agains Wikipedia's summary of
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, which is good enough for me at the moment.)