Symbols - What does heaven look like
The symbolism of incest is complex and needs to be understood in its proper context. It has caused over the years considerable misunderstanding, as later historians took myths designed to explain certain spiritual concepts as evidence that ancient cultures allowed incest. They didn't and the use of Totem animals was one of the means they used to prevent this literally.
In the Poetic Edda poem, for example, Lokasenna, Njörðr is said to have fathered his famous children by his own sister. But he was a god - an Intelligence - a spiritual being.
Please see Mother and Father, for a full explanation.