This sounds like Brother To Demons, Brother To Gods by Jack Williamson from 1979… this is what the cover on my paperback copy looks like:
A range of genetically altered human types are recast as God's and demi-gods and demon like soldiers (
mutant guardians)… basic human stock is considered primitive and are labeled "premen".
Apart from Premen the hierarchy goes Trumen, Mumen, and finally Gods and their offspring.
The first paragraph of Chapter 1 describes the two children and the stories starting point…
Two naked waifs, paternity unknown. A black half god, proud son of Belthar himself. A lovely young goddess, touring the sacred sites of her ancestral Earth. A yelping dog and frightened rat. A red-scaled mutant guardian, its third eye flashing thunderbolts.
The premen are kept on the a reservation and basically in the last stages of going extinct and are at the very bottom of a caste like structure.
Notably they are not siblings… and they're called "Davey" and "Buglet" and they come to notice of a goddess when their dog is killed and Davey threatens them with the myth of the "Multiman"
The "Gods" have their own planets and solar systems as some of them are capable of traversing the enormous distances…
The rise of the Gods by genetic manipulation in an ancient past is revealed through out the book, from the original Huxley Smithwick and his scientists to their first success who became the "Creators".
In the journey they encounter old labs, secret bases and "a botched god" called Pipkin.
The books ends with the revelation
that their child will be the "Ultiman" (the Multiman mentioned by Davey in the first chapter) and bring about a more equal world