It differs a lot in the world but in most western countries and most states in the US being bloodrelated isn't a requirement for incest. Stepparents and siblings counts as well in most instances.
And Patreon (and by extension Visa, Mastercard etc) interpret it even wider and that is what is the standard for most of these VN's.
Well no. In the USA it's not as black and white as you suggest.
It's legal for step siblings to marry in 49 of 50 US states.
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As long as they don't live in Virginia, no part of their relationship is illegal. In all but one state, step-siblings are legally free to get married. Virginia takes a different approach, and step-siblings cannot marry in the state even if their parents are no longer together. Under Virginia law, it makes no difference how long the parents have been divorced or even whether the couple ever lived together as step-siblings.
As for incest. Incest is more complex because the wording of incest laws in the 50 states varies. I am unable to provide a single link that give a definitive answer on what constitutes incest between step siblings.
There are some states where the wording suggests that sex between step siblings above the age of consent may be considered incest, however many state's incest laws with regard to step relationships focus on intergenerational step relations, for example parent or grandparent with a step child, and do not include sexual relations between step siblings if both are above the age of consent.
While sex between a step parent and a minor family member seems to be considered incest in all 50 states, once the step child is an adult, things get a lot murkier. However the opening sentence on marriage seems to indicate that outside Virginia, the ultimate resolution of the question of incest between adult step siblings is not incest "...no part of their relationship is illegal."
