Ranchmom1
For the child, genetically speaking it is only a problem if the family in question has a family history of harmful recessive genes because the child would will have a higher likelihood of ending up with harmful double-recessive traits.
If there are no matching harmful recessives in the parents then there's no more risk than for any other two people.
Legally speaking, it's against the law to have sex with your sibling and certainly illegal if rape was involved.
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Look up incest.