The couple are reportedly going to tie the knot in an intimate non-religious ceremony at their Santa Barbara retreat in front of close family and friends.
Pregnant Angelina is believed to be considering asking her estranged father, actor John Voight, to walk her down the aisle in a bid to make peace with him.
The pair have barely spoken since Voight accused his daughter of having "mental problems" in 2002.
A source said: "Since they started communicating by phone in January, Angelina has thought about asking John to give her away. When she and Brad started discussing getting married, the issue of Angelina reconciling with her dad came up again and again."
Brad, 44, and Angelina, 32, are also going to invite a number of the actor's celebrity friends, including his 'Ocean's Thirteen' co-stars George Clooney and Matt Damon, but want to keep the wedding a secret from them.
The source added: "George and Matt will be going, but other than that it will be a Hollywood-free zone. Brad and Angelina don't want anyone to know it's a wedding until they arrive. Instead they are billing it as a summer barbeque and joint birthday for Angelina and their daughter Shiloh." It was recently claimed Brad's mother Jane has been putting pressure on him to marry Angelina.
A source said: "Jane has a really hard time with them not being married. She feels that they need to be married for the kids. Brad would love to marry Angelina but doesn't want to force her."
The couple have three adopted children, Maddox, six, Pax, four, Zahara, three, as well as 22-month-old biological daughter Shiloh - is believed to be "warming" to the idea of tying the knot for the third time following several conversations with Jane.