[spoiler="Feast For Crows]Umm.. It's pretty explicitly stated in Feast For Crows when the mother is talking to Jaime about their promised rewards for doing it and asking for the return of their people who were at the twins during the Red Wedding (who were dead and weren't in on the Lannister plot)Ralin wrote:It's pretty heavily hinted in the books that she was a Lannister agent, albeit an unwitting one. Remember, her parents were Lannister bannermen.The Grim Squeaker wrote:Random theory - it might be an interesting twist/departure from the books for Rob's wife to be an agent of the Lanisters. The letter thing kinda tips it off.
(The producers did say that the third season would make some big changes compared to the books ).
Spoiler
There was no abortion, but the mother was feeding Jeyne a herbal tea to stop her conceiving, telling her it would help her conceive. As Death mentioned in his post.[/spoiler]
Edit2: Though It would pretty hard for Tywin to actually plan for it ahead of time. I wonder if all his bannermen with pretty daughters were sent instructions for the eventuality.