The Children of Dune miniseries. Specifically between Leto II and Ghanima. In the book, they actually end up getting married, but Ghanima is very firm on the subject: "I will not bear your children, brother." to which Leto replies; "I love you, my sister, but that is not the way my thought tends." Leto, actually, at least claims to be unable to reproduce after forming a bond with sandtrouts.