Very interesting situation. I had something kind of similar come up in my legacy game...
The second generation consisted of Tara and her twin sisters, Bianca and Nina. Tara became the heir and Bianca and Nina were spares. Tara had a daughter Tabatha and a son Max, while Bianca got married and had a daughter named Lilith. It's a matriarchy so Max was out of the running from the start and moved out as soon as he hit young adult. Bianca was evil and was hoping that Lilith would eventually end up as heir if something unfortunate should happen to Tabatha. So even though Tabatha got married and soon became pregnant, Bianca encouraged Lilith to have a child as well. The only problem was from an early age Lilith (who is insane and neurotic but otherwise a lovely girl) kept getting wishes to flirt with/kiss/woohoo/marry/move in with her best friend from high school: Mariana. Lilith married Mariana and moved her into the family house, but how could they have a child together that could be an heir, since Lilith couldn't knock Mariana up and Mariana wasn't part of the bloodline? They needed a male from the bloodline to do the honors...Max. So Lilith's daughter Natasha was conceived by Lilith's cousin, and even though Max couldn't be the heir of his generation he could contribute to the creation of generation four, even though by that point Tabatha's daughter Cynthia had already been declared the official heir, making Natasha a spare like her mother and grandmother.
Isn't it amazing what complex lives these sims can create for themselves? Even though I'm technically the one in control I try to direct their lives based on their wishes and aspirations. It was Bianca's wish to see Tabatha's ghost that made me reason that she'd like to see Tabatha bumped off so that Lilith could take her place. And it actually got much more complicated than that earlier on but I'll save that story for when I get around to updating my legacy story.
Wow that sounds much more complicated than my situation. I have actually never had any sim with same sex tendencies (although I suspect one of the twins my have them, not too many straight guys out there have hot pink as their favourite colour).
As an aside all five can now be found in my profile. Ryan and Ella's traits make them particularly conflicted as they seem to want make messes but make sure everything is tidy at the same time:? .