Does anyone know who is the character in the book that tells the story of her brother being saved from drowning by a slave? They used it in the TV show as Talisa's story, but I remember reading it in the book, being said by an actual character... or so I hope, it's been on my mind for hours and I just can't find anything online.
Talisa's character has been created for the TV show (in the books, Robb's wife is Jeyne Westerling). The story about a little brother saved by a slave from being drowned doesn't appear in the books, it is an explanation of why she started her interest in medicine. Slaves can only exist in Essos (they are outlawed in Westeros), and the river that Talisa talks about is the Rhoyne, but there are no other story like this in the books.
I personally prefer the story about Jeyne Westerling, it is very meaningfull when Tywin pays Jeyne's mother for "her services"...
Jeyne Westerling was the most likely inspiration for Talisa.
Oona Chapman (the actress that portrays Talisa Maegyr) was originally set to play a character named Jeyne, although the character's last name was unknown. (source).
Within a month of the reveal that Oona would be playing a character named Jeyne, a source revealed that the actress revealed her character's name to Robb as "Lissa" from Volantis
In their second scene together, a parting scene, Robb asks Oona’s character for her name and she replies “Lissa” (or possibly Melissa, our source wasn’t sure on this one). When Robb asks what family she is from, she refuses to tell him. When Robb asks where she’s from, she replies, “Volantis.”
Who the heck is Oona Chapman playing?
From an interview with Bryan Cogman, it's clear that Talisa was inspired by Jeyne, and that the characters were changed in production, after the writers' room meetings:
Can’t really speak to what motivated the change as it happened during production, after the writers’ room was done. We did always plan to keep Robb front and center in Season Two from the get-go and we did plan to have him fall in love onscreen and alter the motivation for his breaking of the marriage pact somewhat (as discussed in the question above). But Jeyne Westerling evolved into Talisa as we were starting production.
Ask a GoT Writer: Bryan Cogman on the writing process, Robb and Talisa, and Renly’s peach
It would appear the name change was proposed by Martin as Jeyne would be confusing to book readers:
"[..] if we're gonna have a different character, we should have a different name for her as well. Otherwise, people are gonna get really confused here."
Talking TV With Ryan and Ryan, Episode 12: Talking Power, Magic and Storytelling with George R.R. Martin
From the above, we can see that Oona was intended to be playing a character more similar to Jeyne Westerling, but the character was changed in production, to a different character.
As stated above, the same stories doesn't appear for the books and was fabricated late into the production stage of Talisa's character, however, parts of her story draws from various points of Jeyne' storyline. The story has been suggested (by Cogman in the above interview) to have been fabricated by the showrunners.