What are the critical reason(s) for banning sexual intercourse between blood brothers and sisters(sometimes relatives)?

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Since i was young I used to hear different kinds of taboos and the one which has been exciting me a lot is having sex between brothers and sisters. I have listened from the radio where by the presenters have been discussing a case happened in Tanzania, at a certain street in Dar es salaam where by the relatives are arguing against their relatives ( brother&sister) who are living together as husband and wife ( although not authorized) and they already have two children and expecting the third one. I heard that there are laws in almost every country that forbid brother and sister to get married. Now my question is Are there any scientific reasons for banning this activity? or other reasons apart from scientific(biological) reasons?