Genetically we're all related. What makes incist wrong is a combination of society structure, social construct morality, and birth defect, none of which really apply to synth. Some may say synths are not even alive to begin with and are just robots. Humans are mostly just a bunch of organisms living together forming the body. Our DNA is almost identical to each other, even the synths have different DNA. People need to recall how Danse was found, his DNA matched with founded record of an Institute synth, meaning that synths have different DNA from each other and other humans, and if there are really that much connection between Danse and the SS, the BoS would have found out by comparing DNA as well.