C.T.
Boy, do I ever know where you are coming from. My in-laws were not very happy when we found we were preggo with our second and they actually told us not to have anymore after her. We don't know if we want anymore or not but if we do then that is our decision. They will get mad but then they will get over it. I'd say that the sooner you break the news the better. That gives everyone more time to get over it before the baby gets here. My in-laws were unhappy about it for a while but they did get over it and they love our daughter. I agree with the other answers, let one of your older ones spill the beans or find a cute way to tell them. But, do it as soon as possible. Just my opinion.
C.