A lot of places do not censure incest "Incest between adults is not a crime in all jurisdictions. In France, the offense was abolished when Napoleon introduced his new penal code in 1810. Consensual adult incest is also not a crime in Belgium, the Netherlands, Portugal, Spain, Russia, China, Japan, South Korea, Turkey, Côte d’Ivoire, Brazil, Argentina and several other Latin American countries." (this is between siblings) In the US it is also a problem "How can the United States possibly realize its full potential when close to a third of the population has experienced psychic and/or physical trauma during the years they're developing neurologically and emotionally—forming their very identity, beliefs, and social patterns? Incest is a national nightmare, yet it doesn't have people outraged, horrified, and mobilized as they were following Katrina, Columbine, or 9/11."
There's a popular tag in this manga which is "Uncle x Nephew" but they never said that Ayumi was Kotaro's uncle. They only mentioned that Kotaro's uncle passed away and that Kotaro's mother's relative was Ayumi. This is still incest regardless ;D
http://www.mangago.zone/read-manga/fushidara_na_hanatsumi_otoko/mi/099e3966120641c6472c6cb93c8b6403_chapter-1/7/
Just wanted to clear something up ( ̄∇ ̄")