Symbolism, Imagery, Allegory
The Trondheim to Lusitania trip takes twenty-two years, which is a long way to go without rest-stops or a McDonald's. Due to the wonders of relativistic near-light space travel though, for those in the ship it only takes a few weeks. So when Ender leaves Trondheim behind it means, from his sister's perspective, she won't see him for two decades.
The tricky space-time business does a couple of things. First, it emphasizes difference and transformation. When Ender goes to Lusitania, he's going to a new life—it's like dying and being reborn. He was a piggie; now he's a tree. It's another transformation in a novel that likes its transformations.
The time travel is also a way to emphasize Ender's coolness, and to separate him out from mere mortals. He and Valentine are thousands and thousands of years old; they're ancient and semi-immortal from the perspective of all the other people they meet, and in this way they're both literally living legends. When Ender settles down on Lusitania, it's like he's going from a god to a person—the eternal all-powerful Speaker becomes just another dude, married to Novinha.
Novinha says that if her parents were truly saints, they would come back to her from the dead (1.45)—but then she really does get this heavenly, saint-like guy who steps out of his eternal life to be with her. An improbable outcome, but that's fiction for you.