I must be totally confused...
I thought that Paul Atreides was Ghanima's Father as well as her brother
It may not be true, but I thought Paul somehow impregnated his mother after escaping the city and before they joined the Fremen.
Dune also deals with one theme often reoccurring in his work: incest. As Paul and his mother enter the "Forbidden Zone" (sic), they undress and soon afterwards Paul will find out that his mother is pregnant
This is from a David Lynch site and it may all be a big mistake and misinterpretation of Herbert's book. Mistaken or not it shows that others have had this impression.