"If You Have Sex With An Adopted Or Illegitimate Sibling..."

There was a question I answered a while back on Quora. The question was "If you have sex with an adopted or illegitimate sibling, is that incest?"

Since an "adopted sibling" could mean, among other things, 1) a biological sibling who was adopted by other parents; 2) a non-biological sibling adopted by your parents, I addressed both of those situations in my answer.

There are three definitions for consensual incest: 1) biological, 2) social/cultural, and 3) legal.

Someone who is adopted into your family is, in most places, legally and socially considered your family, and so while it wouldn't be incest biologically, it would be considered as such socially and legally where there are anti-consanguinamory laws and those laws include adopted relations, as laws vary from place to place. These laws are so ridiculous they would apply even if the sibling was adopted into your family as you were both middle-aged adults. In fact, you could marry someone and your parents could adopt that person and make your marriage incestuous. That's how stupid those laws are.

But the laws get even more ridiculous, where they exist. Let's say by "adopted sibling" you mean your sibling that your parents had before you, gave up for adoption, and never told you about. You meet that person completely unaware you had an older sibling, fall in love, marry, even have healthy kids together. (This has happened, by the way.) Then it is discovered they are your genetic/biological sibling. Even though they weren't legally or socially your sibling, they were biologically, and now those stupid laws criminalize your marriage and love. You're actually expected, by the law, to stop having sex with each other.

"Illegitimacy" is a concept that has largely been abolished as far as the law is concerned, at least is the US, where I live. It is also considered an offensive term by many. It means someone was born outside of a legally recognized marriage. In the past, it meant the child might not have a claim to their father's estate, for example. As far as anti-consanguinamory laws go, if your "illegitimate" sibling is your biological sibling, it would still be considered incest in all three ways.

An adult SHOULD be free to have sex with any and all consenting adults. Laws against that are unjust. What someone else wants to call it shouldn't matter.

If people are siblings by birth...

If people are siblings by adoption...

If they are stepsiblings because their parents married...

If they are genetic siblings but raised apart...

If they aren't related by law or genetics but grew up together...

...and they mutually consent to sex with each other, that shouldn't be something to be negatively labeled, shamed, discriminated against, or criminalized.

Siblings have been having sex for as long as there have been siblings, anywhere there have been siblings, regardless of class. There are siblings doing it right now, not far from wherever you are. Some have done it for recreation, some have done it to learn, some have done it for passion, some have done it for love, some have have it because they are living as spouses, some have done it for other reasons. It is their right and they shouldn't have to hide. There is no good reason to deny them their rights.