Translation:Our great-grandfather is the father of our grandparents.
In Esperanto, every letter is separate and has only one possible pronunciation. There are no digraphs, and every letter is pronounced separately, so if there are two of the same letter in a row, there would have to be a syllable split between them and it would almost certainly indicate a compound word.
"si" refers back to the subject (here: "nia praavo"). So
Nia praavo estas la patro de siaj geavoj.
would have to mean
Our great-grandfather is the father of his grandparents.
where "his" means "his own", i.e., your great-grandfather's. That'd probably require a time travel plot à la Predestination (2014).
Predestination is pretty good, but the “bootstrap paradox” (or, if you like fancy words, the ontological paradox) is explored more fully as a premise of the recently-concluded German Netflix drama Dark.
Some people mistakenly call this the “grandfather paradox” because of the paradox of being one’s own grandfather, but that name actually refers to another time-travel paradox: changing the past such that the time travel could not occur in the first place. In the example that gives the paradox its name, the time traveller goes back in time and kills their own grandfather before their grandfather’s child—who later becomes the time traveller’s parent—was conceived, thus meaning the time traveller was never born, so the grandfather wasn’t killed, meaning the time traveller was born, meaning the time traveller wasn’t born, etc.
Both paradoxes are related in that they set up “cause-and-effect loops” and can be “resolved” by positing a time loop that exists as its own causal domain (essentially, a universe that exists only from the traveller’s destination time until the traveller’s departure time, in a closed loop) and has no effect whatsoever on the larger universe, where the time travel never occurred (or, at least, appeared to never have occurred). Since by definition these resolved loops can have no effect on our universe, by the usual rules of what counts as physics, they don’t exist at all, or can only be the subject of metaphysics or religion.