At the end of Children of Dune Leto married
his twin sister Ghanima.
Due to his beginning
metamorphosis into a sandworm and his therefore expected longevity
it is clear that they won't have any joint children. And even if they had those children would never inherit his throne because of
his expected longevity.
These would in my opinion rule out political or genetical reasons for this marriage. This leads me to the question: what was the purpose of this marriage?