The scene in question -- in Dune Messiah, when Alia (then about 16) is practicing against a mechanized target dummy immediately after a bath -- has absolutely no implication of an attempt to seduce Paul-Muad'dib, her brother. In fact, she was surprised by Paul and Stilgar's entrance to her quarters unannounced, and was not expecting company at all. While she does flaunt her nakedness for several minutes into the conversation, it's more in defiance of their intrusion upon her quarters than anything remotely seductive.
That said, the early Dune series does contain several references to endogamy. The Bene Gesserit deliberately hid Jessica's Harkonnen ancestry from not only her, but everyone else, because, had she bore them the daughter she was supposed to instead of Paul, it was their intention to cross that daughter with Feyd-Rautha or some other similar Harkonnen scion. That child was supposed to be the Kwisatz Haderach.
The Bene Gesserit are, in their desperation to preserve the Atreides bloodline in a way they can control, similarly driven to consider trying to contrive an incestuous relationship between Paul and Alia, but neither of them ever know about this directly, and neither collude in it as the quote above suggests.
It is correct that Leto II and Ghanima discuss the possibility -- and both reject it as repugnant -- as a way to seal the dynasty, Egyptian style. That said, they do marry, in the legal/religious sense, and their empire is left to assume that any children Ghanima produced were Leto's. In truth, however, Farad'n becomes Ghanima's concubine and secret father to their genetic line; Leto II is never actually capable of reproducing, since he accepts the sandtrout-skin before hitting puberty.
(This timeline is altered somewhat in the television adaptation by John Harrison, almost certainly in part because of the difficulty of dealing with sexual themes where children are concerned. In the book Children of Dune, Leto II and Ghanima are 9-10 years old! In the miniseries, they're closer to 18 and being played by young adult actors.)
After that, the subject never really arises, as we never again see close relatives in a position to involve themselves thus. It's possible that Leto II contrives at incest within his own breeding program, but it's never explicitly mentioned.
