His character Robert McAllister allegedly will not be killed but simply written out so that there's a possibility he will come back in the future.
The rumor that Rob Lowe is quitting "Brothers & Sisters" has been confirmed by ABC president Steve McPherson. He released a statement to several websites such as Deadline Hollywood which was first to report the news, saying "We would like to keep Rob at ABC; we feel he should be anchoring his own franchise. We are in talks now."
McPherson won't speak further about the reason of Lowe's exit as well as the new project. But rumor has it, Lowe feels that he has been "underutilized" in the drama's storyline since the show changes four show runners to the least. For one, Lowe's Robert McAllister was brought in to the show when the series suffered from a rating drop. His addition brought up the number and he was kept as a regular since.
Nevertheless, insiders told E! Online that the split between Lowe and ABC is amicable. The network has been talking about killing off a character to minimize the budget and Lowe feeling "underutilized" seems to be an answer to their problem.
There is no clue yet when and how Robert will be sent out of the show. However, it is likely that he will follow Balthazar Getty's move from being a regular to recurring character. This means, Robert will not be killed but simply given an exit story so he can come back later. [Spoiler Alert] The crack in Robert and Kitty's marriage allegedly will be the reason since the man whom Kitty had an emotional affair with, Alec Tyler, will be brought back into the frame.