Zap2it.com is reporting that when the 5th season of Brothers & Sisters opens, viewers are going to find the marriage of Justin and Rebecca at a very troubled point. Justin wanted to do medial work in Haiti for a year but Rebecca thought that was too long for them to be apart so early in their marriage. With the one year time jump the 5th season is going to be taking, we learn that Justin did go to Haiti.
Rebecca's departure will also bring some closure to her some times rocky relationship with her mother, Holly(who is rumored to have suffered memory loss as a result of the events in the 4th season finale). Rebecca's departure doesn't neccesarily mean the end for Holly's character. The discover of the Ojai water geyser is sure to provide plenty for Holly to do.
Rebecca's exit storyline is schedueled to take place in the 3rd and 4th episodes of the 5th season and as we get closer to those airdates spoilers should continue to surface.