For those of us who read the book long before the movie, it is sometimes hard to separate the sources and remember how much is fully explained in the movie. So this explanation might have some parts not fully explained in the movie.
All three siblings were considered for the Battle School leadership programme. Peter and Valentine had both been rejected before Ender entered the program (and the book makes clear that Ender's existence may have been influenced by both their potential and failure as third children were normally not allowed but the potential shown by his elder siblings suggested a further experiment might be worthwhile). His childhood experience was certainly influenced because he was a highly frowned up third and subjected to abuse because of it.
It is clearly stated in the book but only hinted at in the movie that Valentine was rejected for having too much empathy so would be too "soft" and that Peter was rejected for not having enough empathy and was too cruel. Ender struck a better balance having enough empathy to understand the enemy but enough ruthlessness to fight to win (though the school still has to trick him to make this work in the real battle).
Other parts of the sibling relationships are far clearer in the book but hinted at in the movie. Peter is prone to cruelty and tortures his siblings a little (alongside other acts of cruelty). This somewhat enhances the bond Ender has with Valentine as she is sympathetic to Ender and offers him confort when needed. Hence the bond and hence her ability to persuade Ender to go back to the school. His love for her is part of what motivates him to continue the program so he can fight the formics ("buggers" in the book) to save her and the planet. Ender also has a persistent fear of turning into his cruel brother.
Both his siblings play a big part in creating Ender's complex character. The movie hints at much of this but is far less explicit in showing it than the book.