If you think the twins are especially interested in ethical nonmonogamy, you are wildly misreading them. Greyhide and X'z are easy lovers to pick up basically because they respect the marriage.
It's less me thinking the twins are 'especially interested in ethical nonmonogamy,' and more so me thinking that it would merely serve as a fun unexpected convenience if both Rowan and Alexia were drawn to one twin or the other, being united in that desire (by the player's choices lining up to it for this potential branch). I don't believe the ethicalness of it would really matter one way or the other to the twins. Or, rather, the idea that there
even is an ethical or non-ethical way of going about having a couple that catches their interest falling for them. The concept of that consideration doesn't really seem to be a part of their worldview or philosophy on such relations.
From what I've understood of Jezara, in particular, the prospect of Rowan & Alexia both submitting to her together would feed into her outlook of the ideal being unrestrained pursuit of one's core carnal desires; that one is their true self in that pursuit. With this being two people she has interest in and use of, it would make it all the more appealing to her; in-fact, I'd imagine it would be quite satisfying having them both eager to serve and devote themselves to her. Moreover, Jezara isn't beyond having a hand in molding or guiding one's core carnal desires for those of interest to her to play into her own desires and pursuits. An on-going mutually satisfying relationship that furthers either her goals or desires seems to be her endgame with her relationships. With the caveat being that she shapes or directs others' core desires to want things that she herself finds pleasure or use in.
This all aside, that this hypothetical branch is a mix of a
twin's influence being 5 and a >50 relationship rating between Rowan & Alexia, it's not as if the context doesn't include Jezara leading them to this route in such a way as we see she does with Alexia. Regardless if she would lead them individually or not, the result would be the two of them finding themselves with an overpowering desire to be with and serve Jezara. Them doing it together or not simply would capture different flights of fancy for Jezara. Personally, in my perspective, it just seems like it would have more benefits for Jezara if they both fell for her together, and lead to some rather indulgent, smug-inducing interactions that would be up her alley. It wouldn't line up with what we see from Jezara for her to be bothered by them both mutually being devoted to her. Likewise for Andras.
When I originally brought up Greyhide, I mentioned the context being different for Rowan & Alexia and the twins vs. that of them and Greyhide. This is in part what I mean. The only similarity is that it would be a polygamous setting, but as I said, the context of the relationship would be completely different, naturally. I wasn't commenting on any particular set of ethics that one or the other may have in regard to it. I don't really think such a thing has a factor in such a route either, as it would be a route where the twin of the player's choice succeeds in gaining influence and manifesting desire in both Rowan & Alexia.
The difference from what's currently a branch in the game is that in this supposed branch, Rowan's & Alexia's own relationship is in good standing despite both being overtaken by desire for the twin in question. As I explained, many players are going to find themselves going down this choice path, regardless if there is content for it or not, as it's just a common natural inclination for many players to both want to pursue a twin and want to make positive relationship choices between Rowan & Alexia. A large portion of players aren't going to presume they have to purposely sabotage Rowan & Alexia's relationship to get a path with the twins going. Nor does it seem unreasonable to presume there would be a path where both Rowan & Alexia submit to a twin, and for there to be content that goes in line with this. Not everyone is going to be using a walkthrough to guide them either, as, understandably, many people want to play blind.
So, it just, as I said before, would be bringing something to an avenue many people are going to find themselves down that is currently vacant. Like, currently, there isn't any pay off for a player having both a high relationship between Rowan & Alexia, while also trying to pursue a twin oriented route. The only way you get a pay off currently, is by purposely doing things to hurt their relationship. There isn't currently a branch were you can both pursue submission to a twin while also having high relationship between Rowan & Alexia. It is this gap that this proposed branch would be filling for, if it came to be.