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.
As a child, Ender is bullied at school for being a "third", in particular by a bully named Stilson. After Stilson engages him in a fight, Ender beats him thoroughly and also ends up killing him. At home, Ender is tormented by his brother Peter, a sadistic bully who jealously resents the attention Ender gets from the military. His only refuge is with his beloved sister Valentine, who acts as his protector and only friend. When he is accepted into Battle School, he is broken-hearted at the thought of leaving her and so is she, but she assures him that they will always have a bond.
And in the dream he saw Valentine not Petra because he's so protective of Valentine rather than Petra. I think Ender did not like Valentine in any different way other than his close friend and a confidant.
I definitely got that vibe too. The book only seemed to confirm it on page 189. As taken from the book:
As always, the serpent waited in the tower room, unraveling itself from the rug on the floor. But this time Ender didn't grind it underfoot. This time he caught it in his hands, knelt before it, and gently, so gently, brought the snake's gaping mouth to his lips.
He had not meant to do that. He had meant to let the snake bite him on the mouth. Or perhaps he had meant to eat the snake alive, as Peter in the mirror had done, with his bloody chin and the snake's tail dangling from his lips. But he kissed it instead.
And the snake in his hands thickened and bent into another shape. A human shape. It was Valentine, and she kissed him again.