The problem created in your question is the definition of "Step-Siblings" which is not the always same as a "half-sibling". Some step-siblings may be Half-Siblings as well, but not always, and rarely do they imply the same thing. Half-Siblings share a common A parent but not a common B parent. If we say that two step siblings, S_1 and S_2, exist, then S_1 was the child of Parent A_1 and B_1 and S_2 is the child of Parent A_2 and B_2. They become step-siblings if and only if after A_1 and B_2 (alt B_1 and A_2... of for the really progressive, A_1 and A_2 or B_1 and B_2) engage in Common Law Marriage. Because neither sibling have a common parent, they are not genetic siblings and only Siblings-In-Law. A half sibling (S_3) will be any child of A_1 and B_2 from any point regardless of Common Law Marriage status between the parents (if before it was after the S_1 and S_2 but before Common Law Marriage, they could count as a Step-Sibling technically).
Most familial marriage laws are based on Blood relations (these days, interpreted as genetic relationship) not legal marriage status, though usually royalty like to block the child of the king's mistress from the throne (in Europe at least, but this part is tangential. If you have possible claim to a throne, please amend jurisdiction.). So
two Parents-In-Law are legally allowed to marry. Similarly, you are legally not allowed to marry Half-Siblings but you can marry a non-Half-Sibling Step-Sibling (Or just about any In-Law since they are not blood/genetic relatives.). There is nothing legally wrong with the Brady Children (upon reaching age of consent in CA (18), obviously) from marrying their counter-part sibling in the modern legal sense... though anyone can tell you there's a hell-a-lot wrong with it in the non-legal sense of the world... also everyone can marry Alice. That's why she's in the middle of the Brady Tic Tack Toe Board.
That said, it's still considered a small taboo to portray children who grew up together under the same marrying, even if they are only step-siblings. Parents-In-Law hooking up with each other is much easier to portray as not only does it hold the squick factor of a family romantic relation as well as the always funny when it's not you of your elderly parents talking abut a sexual relationship with someone. Most people are content in the knowledge that your parents had sex exactly X + 1 times in their entire life (where x is the number of your own siblings of siblings) and don't like to think of them as having a sex life at all by the time they have adult children (The crux of the humor of the How I Met Your Mother). Homer's comment about the freak children he and Marge could have is funny because Homer and Marge are not products of Abe and Marge's mother (I know she has a name, I just cannot remember it). Homer is voicing the real reason that why blood relatives are not allowed to marry, thinking it applies to blood family and common law family equally. Homer's genetics will not be screwed up if his father dates his wife's mother, though they are pretty screwed up to begin with. This is not the same if Bart and Lisa were to marry (as adults, anyway... of course, insert joke about how Bart's been a decade old for three decades).
Again, eyes of the law only allow this. It's still seen as sort of taboo. As one personal note, there was a point in time where a family member of mine was shipping me with my Aunt-In-Law (My Mother's Brother's Wife's Sister) which again,perfectly legal... but it's rather difficult for someone to hear you introduce yourself as "My Cousins' Uncle" and not think of West Virginia. This didn't help to the person who wanted to make it a thing, though it was hardly the only issue. But were I to write a screwed up extended family comedy, this scenario would be a great source for a laugh.
EDIT: To summarize, lets take the Brady Bunch. Boys were from Mike's first marriage (Wife's name unknown, Will be referred to as Brady-Doe pairing). In series, Mike was a Widower and Carol's situation was never discussed but she was intended to be a divorcee to a man with the surname of Martin (ABC refused to put that on air. Keep in mind that the Brady Bunch was the first TV series to show a married couple sleeping in the same bed... at the same time... IN PJs! Divorce was not TV safe back then). This will be referred to as the Martin-Tyler paring.
All children in The Brady Bunch are products of a their biological parents first marriage. All the Brady Boys are Brady-Doe and all Brady Girls are children of Martin-Tyler. As detailed in the theme song (don't make me copy the full song, I already spent way too much time reading about Carol's pre-Brady backstory to get her maiden name and the name of her first husband. I'm not singing a tv theme everyone knows by heart.) the for men and for women became a family after the marriage of Mike and Carol, which formed the Brady-Tyler marriage and which produced no offspring (Brady-Tyler children).
Because of this, the Brady-Tyler family consists of the married Mike and Carol and 3 Brady-Doe boys and three Martain-Tyler girls. As familial marriage laws exist because reproduction among genetic siblings is very likely in a child with all sorts of genetic diseases rather than a reproduction among two family lines with no significant family relations.
As such, a child of a Brady-Doe would be allowed to marry a child of Martin-Tyler even if they are now step-siblings in Brady-Tyler, as genetically. The fact that Mike and Carol are now married does not stop a marriage of Mike's (Adult) Son and Carol's (Adult) Daughter (Who are Step-Brother and Step-Sister to each other, respectively). But for marriage of Mike and Carol, they would not be any kind of legal family and in all cases, they are genetic family, which is what the law is preventing.
For bonus points, Cousin Oliver is Carol's Nephew, so her daughters are legally barred from marrying him, but any rule 34 (If it exists, there is porn of it) search will likely yield a Slash fic relationship of a Brady Boy and Cousin Oliver incorrectly labeled as incestuous... they are not genetically related... Though again, I refer back to earlier comments about being the first TV series to feature a man and a woman sleeping in the same bed.
The order of which marriage (the parents' or the kids') does not matter in this case because it does not affect the blood relationship of the kids and the parents. Greg and Marsha are not genetically related at all, thus are not tripping incest laws.
As for part two, you should probably look up marriage law, family law, and incest. Also for gagging, should have a specific jurisdiction (Nation or state (Especially in the U.S. because many laws differ between U.S. States)).
