Legal Question in Family Law in California

Your brother has no rights. The rights belong to the children. The court system's obligation is first to encourage parents to cooperate and reach agreement on a parenting plan, and if they cannot do that, then to issue orders that are "in the best interests of the minor children." All he can do is assure that the life he offers the children when they are with him is the best for them and then convince the court that that is true. Whichever parent is the most persuasive that they will be the best for the kids is the one who "wins."