I've been searching for a House of the Dragon fic for about a week now. i'm pretty sure i read it on this platform.
Here is what i remember:
The OC was the younger daughter to Viserys and Aemma, by at least a few years. It was one of those stories where Viserys and most of the court would have rather her be the heir instead of Rhaenrya but because she was younger Rhaenrya was still the heir. I believe it was to eventually be a Daemon/Oc but i could be wrong.
I do know that she was everything Rhaenrya should have been after being named heir, even getting along with her half siblings and doing her duty to be married.
She married after Rhaenrya married Laenor. I believe she married a Tyrell but was secretly in a 3 way relationship with his sworn guard as well, whose name was Sebastian I believe (her new husband was bisexual). They did their duty and had kids. Her husband had been battling a mysterious illness for years and the last chapter I remember reading was of his death.
i believe that Rhaenrya was eventually going to be disinherited and the OC was going to be named heir but i didn't get to that part. I had thought i saved it to my reading lists but i just haven't been able to find it.