Yep I know they were siblings but, throughout Ender's Game Ender only told Valentine what he feels. He saw her in a game. Why not Petra? And I can feel that he loves her in a different way or maybe I have to read the book. So did Ender love Valentine in a romantic way?
The answer is "No". My personal interpretation is, Ender and Valentine are close to each other like normal siblings. Since the beginning of the movie, we were shown that Ender shares everything with his sister because he trusts her most.
Check this to know more about Ender's Character.
And in the dream he saw Valentine not Petra because he's so protective of Valentine rather than Petra. I've seen this movie but I didn't feel like Ender liked Valentine in any different way other than his close friend and a confidant.