I don't think it was actually canon in the books. It was definitely very much implied in the movie, but I thought in the books they technically married and *told* everyone that Ghanima's children were also Leto's but they were actually Farad'n's. Because Farad'n was to Ghanima kind of like what Chani was to Paul. So it was, but it wasn't, because they didn't actually do any of that married couple stuff ;)

Unless, of course, I'm remembering wrong, which is possible.