@StarFleet Carl
Steve seems extremely unlikely to cheat
In his case, it's actually difficult to cheat since all he has to do is ask Jasmine for permission.
I meant, in the absence of Jasmine, with a more traditional girlfriend, Steve would be very unlikely to cheat. So, Angie right now is large a possibility only because of Jasmine (on the assumption that Steve would simply date someone else if there were no Jasmine, which - at this point - seems plausible).
And, yes, no one in their immediate family is biologically related, at least as far as we know, and the odds of an unknown connection are infinitesimal. Of course, so are the odds of getting a do-over :) But, that would (as awnlee jawking says below) tend to make it (more of) a soap opera. It might also cause some interesting and awkward conversations once DNA testing is commonly available :)
I alluded to their lack of biological connection way back in the Book 1 Foreword with a note to there being a potential combination which would NOT trigger another tag. The 'incest' tag specifically mentions blood relations; it wouldn't apply (which makes any 'gray area' things that have already happened correctly tagged as well).
In some states they could legally get married. In Texas, the laws around incest specifically include adopted persons without regard to age. At no point could they legally get married in Texas. And, if they they had sex in-state having gotten married somewhere else, they would technically be violating the law. Of course, so are Mel and Cammie, Lizzie and Janet, Andy and Cal, and so forth, so ...
For reference, the current Texas law is:
Texas Penal Code ยง25.02
(a) A person commits an offense if the person engages in sexual intercourse or deviate sexual intercourse with another person the actor knows to be, without regard to legitimacy:
(1) the actor's ancestor or descendant by blood or adoption;
(2) the actor's current or former stepchild or stepparent;
(3) the actor's parent's brother or sister of the whole or half blood;
(4) the actor's brother or sister of the whole or half blood or by adoption;
(5) the children of the actor's brother or sister of the whole or half blood or by adoption; or
(6) the son or daughter of the actor's aunt or uncle of the whole or half blood or by adoption.
Getting the adoption rescinded would do nothing, even if that were a thing (which I doubt it is), since Angie's the daughter of Steve's uncle, and that clause mentions adoption again. Yes, Frank didn't adopt her, but I don't think that technicality would amount to much, especially in 1982 where a paternity test would be difficult to impossible.
That's the 2022 law, but I did check (without saving the reference, alas) and it was the same in the 1980s with regard to adoption.
Jane did mention that she's very hard-pressed to see anything wrong with it given 1) their unusual ages and 2) lack of biological relationship - but it would still be illegal and she's still a mandatory reporter (which doesn't apply to e.g. Angie and another girl, but would apply to incest - the Texas Sodomy Law wasn't under mandatory reporting, nor is sports betting).
Mandatory reporter status is pretty much secondary at this point, anyway; the real bind is Jane's obligation to Helen (specifically around the two of them having sex) and neither Steve nor Angie wanting to undermine that. However, it's another reason Steve (and, presumably, Angie) have for keeping on the right side of the Romeo-and-Juliet clauses in the law - if either admitted to having sex with someone outside the allowable age range, we're back in mandatory reporting territory.