Actually the true definition of incest is sexual relations between close blood-related family members such as mother/son, father/daughter, brother/sister, aunt/nephew, uncle/niece, grandparent/grandchild and first cousins as these are the types of sexual relationships that can cause horrendous birth defects. Second cousins and beyond are not considered closely related as the chance of birth defects is the same as if they are unrelated. Step family are not blood-related it's not incest, just because some religious figures with a stick up their ass says it's incest too have sexual relations with them doesn't make it true.
And if you really want to get technical all humans beings in one way or another if you go back far enough are all related.
Nope, marriage and adoption are both counted as incest, hence why step siblings or adopted children can get in just as much trouble as blood siblings or children. It is often looked at in regards to fiction, such as when dealing with the distribution rules on certain websites like Patreon, as a way around said rules, but it is considered incest in almost every law against such relations on the planet. The few countries that don't ban step sibling or step parent incest are usually the same ones that don't ban loose blood relations like cousins. That said, not all countries that allow loose blood relations allow step relations. A prime example is my own country, step siblings having any kind of further relation than that of siblings is illegal here, the same for parents. Now, some areas only target marriage, one has repealed their criminal statute altogether, and some only go as far as distant blood relation, but there are laws here against incest between step relations. Examples include Alabama (Stepchild or Stepparent), American Samoa (Stepchild or Stepparent), Arkansas (Stepchild or Stepgrandchild), Colorado (Stepchild), Connecticut (Stepchild or Stepparent), District of Columbia (Stepparent), Georgia (Stepchild), Illinois (Stepchild or Stepparent), Kentucky (Stepchild or Stepparent), Maryland (Stepparent), Missouri (Stepchild), Montana (ANY step relation), Nebraska (Stepchild while a minor), New Hampshire (Stepchild), North Carolina (Stepchild), Ohio (Stepparent), Rhode Island (Stepparent), South Carolina (Stepparent), Tennessee (Stepchild or Stepparent), Texas (Stepchild or Stepparent and this continues even if the relation ceases), US Virgin Islands (Stepparent), Utah (Stepchild or Stepparent, but is nullified if the relation ceases), West Virginia (Stepchild or Stepparent), and Wyoming (Stepchild or Stepparent). Some of these examples also include relations by adoption and there are some instances of loopholes as with any law, hence why I listed specific banned relations in parentheses as well as any specific conditions to that ban if any apply. Also, don't bother asking why some bans are targeted at only one side of a pair a ban applies to, the child and not the parent for example, I don't know why, I just follow the laws, I don't make them. In regards to games, that is acceptable in my country as games are protected by the first amendment, however, they are still subject to the rules of any website or company involved with them. This is where the issue with sites like Patreon comes in as their rules specifically ban blood relations, not step or adopted relations. Long story short, it is how the party involved defines it that determines if it is incest unless they leave it undefined, in which case it is by default. In this case, I'd have to hear a more knowledgeable source about how the incest tag is treated on F95Zone, though I have seen it applied to step or adoption relations in other games.