nothing physically incestuous ever happens between leto and ghanima but nevertheless incest is written all over their relationship. for one thing they're husband and wife, obviously, but it's more than that. it's the way they casually discuss, though quickly reject, the idea of continuing the atreides line through themselves, with each other (ghanima telling her brother, "i won't bear your children"), and the way leto refers to her tenderly as "ghani." even long after her death he does this. stuff that makes you raise your eyebrow a little.
but most of all for me it's that exchange that leto has with farad'n at the end of children of dune. he emphatically, even triumphantly reveals that there will be no betrothal between farad'n and ghanima as farad'n had been expecting there to be, because "[his] sister will marry [him]." and then, even while telling farad'n that he (farad'n) will father the continuing atreides dynasty, he makes an explicit point to knock farad'n down a peg: "note this, cousin harq al-ada. this is the way it will always be with [ghanima and i]. we'll stand thus when we are married. back to back, each looking outward from the other to protect the one thing we have always been." / "remember that, cousin, when you're face to face with my ghanima. remember that when you whisper of love and soft things, when you are most tempted by the habits of my peace and my contentment. your back will remain exposed." oh and also he is described to smile "mockingly" at farad'n in between those two sentences. idk. he shows such a... possessiveness, for lack of a better word, toward ghanima during this scene. it comes across as him wanting to set clear boundaries with farad'n regarding ghanima, a la "you will be her paramour and father her children because i no longer can, but i am her husband and i am her brother and i am the other half of the single whole that she is part of." like i do not think we are at all intended to read leto and ghanima's relationship as purely brother/sister
