If a brother and sister have babies, the risk of them passing down genetic disorders and problems increases significantly, as there are some genes that may be recessive (weak) in both brother and sister but they may become active if both of them pass it down to the child. Additionally, if a brother and sister have babies it can also lead to social problems and they may be ostracized, and there may be problems in the child’s future as well, given that they are a product of an incestuous relaitonship.