I don't know how specific to be here so I'll start kinda vague. Neria was born in clan Sabrae, Templars (who had been wearing plain clothes) tried to take her and her twin brother when they were playing near the edge of the forest. Their father was nearby and tried to protect them but the Templars killed him and took Neria, her brother managed to escape. A few months later Neria's mother and brother left both the clan and Fereldan, her husband was dead and as far as she knew Neria had been taken by multiple human men. She had no hope that Neria was still alive, so she moved to a village in the Anderfels, hoping to keep her son safe. Years later she falls in love with and marries a city elf, and they have Gwyneira.
The only reason Gwyneira and Neria end up meeting is because Isabela took one look at Gwyneira and thought that, she really kinda looked like Neria. Isabela mentions it to Zevran the next time he visits, thinking it's a weird coincidence. Neria had already gotten multiple letters from people saying how much Rook looked like her, so she decided it wouldn't hurt to see for herself. The first time they meet they don't figure it out, yes do look alike but that doesn't necessarily mean anything. But then Gwyneira writes home to her mother about meeting the hero of Fereldan and how she kinda looks like her, and her mother writes back asking to meet her as soon as possible. So Gwyneira uses the Eluvians to bring her mother to the hall of Valor to meet Neria, and then mother and daughter see each other for the first time in over thirty years.
The thing with Gwyneira is she can trust someone completely but still doubt they care about her. So when Neria tells her it would be best to not let the Crows know that they are sisters, she agrees. Things had already been awkward with Lucanis anyway, besides the only one her pulling away would hurt is herself, right?
Maybe Lucanis asks around and Taash accidentally lets it slip, or at least says enough for Lucanis to know that whatever changed with Rook, happened at the Hall of Valor. Or Zevran decides keeping this a secret from the Crows won't really change anything and sneaks into the Lighthouse to give Lucanis the "What are your intentions with my sister-in-law?" talk.