A DNA test had revealed in previous weeks that they - Justin (Dave Annable) and Rebecca (Emily van Camp) - were not biologically related.
But by then, they had grown close as siblings: Rebecca was brought into the Walker family when they thought she was their long-lost sister, the product of an affair between their late father and Rebecca's mother.
As the story line has unfolded in recent episodes - first with hints and flirtation, then with confessions - fan reaction has been divided between "yay" and "ick".
"I don't want people to feel that we're not sensitive to the issue of incest," said Alison Schapker, the executive producer who runs Brothers & Sisters with Monica Owusu-Breen.
"We look at this story as two people falling in love," Owusu-Breen said.
"And they aren't related. Our minds are wrapped around that, so we're looking at this as a love story?" she added in up-speak.
"With complications."
Perhaps surprisingly, given that part of last season and much of this one involved integrating Rebecca into the Walker clan, Schapker said the romance had been in the works for "an awfully long time", and the writers "actually have a big plan for Justin and Rebecca that involves the series as a whole".
Still, jokes are inevitable.
"We do reference Flowers in the Attic from time to time," Owusu-Breen said, citing the famously and bizarrely pro-incest V. C. Andrews Gothic novel.
The Rebecca-Justin coupledom will roll out in season three, and Owusu-Breen and Schapker said the writers would be mindful of the audience as it does.
"As far as some of the more strident criticism of it, my hope is that we're having a big old Walker fight with our fans," Schapker said, referencing the messy-but-loving fictional family at the centre of Brothers & Sisters.
"Just the way this family can get into some knock-down-drag-outs and yet come back together, I'm hoping we tell the story in such a way that everybody can get on board with it.
That's kind of my fantasy." - Kate Arthur
