The Zindagi Na Milegi Dobara director has managed to convince the siblings to portray the lead roles, the Mumbai Mirror reports.
A source said: "Both Ranbir and Kareena were always aware that they couldn't pair romantically for any film. So for them to come together for any project, it needed to have an extraordinary script.
"Zoya narrated the idea to Kareena while she was shooting for Talaash. Kareena immediately loved it. Ranbir too has given it his consent. The film will now go on floors by the end of this year."
The film reportedly explores the relationship between a brother and a sister.
"Zoya will be reviving the brother-sister relationship genre. And she has now managed to cast real-life siblings Ranbir and Kareena for it," added the insider.
Imtiaz Ali was keen to cast Kareena in Rockstar, which stars Ranbir Kapoor, but was unable to do so since they are first cousins.