According to California Law "stepshit" as you call it isn't actually illegal just so you are aware. A few pages ago I list what constituted incest as defined by California laws, if you are wondering why I used California laws specifically because its based there and for all intents and purposes in some states you can fuck your family members as long as you don't make babies because reasons.... fuck if I know America has some weird laws on a state by state basis.
Exert for the wiki on laws in america
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has repealed its criminal incest statute,
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and only criminalizes incestuous marriage.
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"targets only parental figures",
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and
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does not apply any penalties when both parties are 18 years of age or older
Edit/ PS: Some states do have step relatives as being illegal while others do not if this is what you are implying then if I'm not mistaken California is one of the states that does not imply that as being illegal if it did then a lot of the weirdos in hollywood would be in prison.