Marrying a close relative was never allowed or permitted in the Bible, so looking at some important information of marrying a host sibling, which is not acceptable, it's a big thing. Marrying a close relative can increase diseases and genetic failures that might arise.
Leviticus 18says that none of you shall approach you or any as near of kin to him to uncover their nakedness. I am the Lord.
This shows that it is prohibited to even have any sexual relationship with any close relative. And science also shows this information that if you marry anyone that is so close or nearest to you, it is not something that is good.
You might end up having babies that does not meet the genetic requirement or they might be deficient in one aspect or the other. Mentally, physically, if you accept any advice of people marrying in the same way or the same family, then it is very wrong and not acceptable biblically.
