...there's no cultural consensus that an Old Suitor is automatically to be preferred to a New one, or vice versa. It's not difficult to call to mind multiple examples of Hollywood movies in which it's right and proper that a New Suitor should displace an Old Suitor...If one were to transpose these "Old Suitor/New Suitor" criteria into familial relations, one would get something along the lines of "primogeniture" (the firstborn's right to inherit from the parents, and, by extension, all other privileges descending from that status) vs. "ultimogeniture" (the exact opposite re: privileges being conferred to the youngest-born). But age is only sometimes a criterion. Often it has more to do with being first to "call dibs," as it were.
A typical example of "right relations" being governed by the "I saw him first' principle appears in the 1960s comedy teleseries I DREAM OF JEANNIE. As the story goes, modern-day Air Force pilot Anthony Nelson opens an antique bottle and frees a beautiful genie named Jeannie, who then schemes for the next four TV-years to get Nelson to marry her.
In the third season the show introduced Jeannie's sister, also called Jeannie, distinguished only by brunette hair. I don't believe that it was ever established which one was older-- maybe it hardly counted once both of them passed 2,000 years-- but regardless, Jeannie II's attempts to move in on her sister's territory was a clear example of alloting right relations by virtue of Jeannie I being "first in line."
In contrast, from the same time-period, we have the teleseries THE ADDAMS FAMILY. Unlike JEANNIE, this one starts out with a kooky couple that's married from the first episode: i.e., Gomez Addams and wife Morticia. However, in the show's second season, a two-part episode from 1965 revealed that Morticia also had a near-twin sister, Ophelia.
In this flashback tale, it's revealed that Ophelia actually "had dibs" on Gomez, in that their respective families had arranged a marriage between the two. Ophelia was willing to marry Gomez but was also too dizzy to really feel anything about him one way or the other, while Gomez was frankly terrified of the dizzy broad. However, on first sight he falls in love with her milder sister Morticia, and she with him. The comic scenario in which the lovers who are destined to be together is only possible because they have to find some way to get around Ophelia's privilege as both fiancee and (I believe) older sister.
So in both of these examples, we're dealing with a "true love" and a "false love," irrespective of which female character meets the male character first. However, a complication comes up when there's something along the lines of a "No Exit" situation, where three characters are destined to remain apart.
DC Comics' original ANGEL AND THE APE series from the 1960s was a lightweight comedy series about a girl and a gorilla, two detectives who solved weird crimes. Aside from the following house ad, which carried a strong "King Kong" vibe, Angel O'Day and Sam Simeon (guess which is which) had no relationship beyond their business partnership.
A 1991 mini-series by Phil Foglio revived the concept with a little more sexual interplay, and also added a new continuity-wrinkle: lady detective Angel was the sister of another 1960s DC character, "Dumb Bunny" from the INFERIOR FIVE series. In one issue of the mini-series, the sister with the questionable intellect reveals that she's interested in Angel's partner.
Then, a little later, Sam the Ape has a confrontation with long-time super-villain Grodd (his grandfather, actually), and this results in the revelation that Sam doesn't have any feeling for Dumb Bunny, but that he does carry a torch for his partner Angel-- who certainly doesn't have any reciprocal feeling for him.
This does cause Dumb Bunny some aggravation for a time, though by the story's end she's shunted off to a more acceptable romantic interest (who is at least of her species...) There's no indication that "the one who saw him first" will end up with Sam, possibly because of that whole "not-the-same-species" thing, though Angel and Sam are still partners at the end of the mini, which to my knowledge remains non-canonical in DC continuity.
So does "clansgression" in the sense of "sister-competition" over a male even exist here, given that one sister wants Sam for a boyfriend, but doesn't get him, while the other doesn't want him, but does remain at least in his company? I would say that Foglio is toying with the more normal trope seen in the previous examples, but has deliberately flummoxed the pattern because he doesn't really want to depict an interspecies romance. Yet one may still say that clansgression exists here in the same I said it could in Wilkie Collins' novel THE MOONSTONE:
My verdict is yes, but with the qualification that the MOONSTONE's "incest" is only transgressive-- and clansgressive-- *in posse.* Because a unison of two near relations of roughly the same age strongly *suggests* a unison between blood-siblings, the basic situation of a sexual relationship between cousins will always carry a potential for transgressivity, no matter whether the author makes use of that potential or whether the audience recognizes it.-- CROSSING THE LAWLINES PT. 4.
If Sam had been a human character, I would expect that in a roughly similar scenario Foglio might have left the door open for some future liaison between the detectives-- in which case the sibling-rivalry would have become *in esse.*