From A Wiki of Ice and Fire
|Book||A Feast for Crows (mentioned)|
Saranella was a septa of the Faith of the Seven in service to House Lannister when Cersei Lannister was a young girl.
After Melara Hetherspoon drowned, Cersei Lannister asked Septa Saranella the meaning of the word valonqar. She responded that it means "little brother" in High Valyrian. 
A Feast for Crows
Cersei Lannister tells Taena Merryweather about Maggy the Frog, telling her that Tyrion Lannister is the valonqar and that it was Septa Saranella who had told her the meaning of the High Valyrian word for "little brother".
- ↑ A Feast for Crows, Chapter 39, Cersei IX.