Complex sentence: As riders, they may be better than some of the other teens, if not for their sibling rivalry, which usually translates to opposite commands being given to their dragon, resulting in very chaotic flying/crashing as the dragon tries to follow both commands simultaneously.
Split into simpler sentences: As riders, they may be better than some of the other teens, if not for one problem: each other. Their sibling rivalry usually translates to opposite commands being given to their dragon, resulting in very chaotic flying/crashing as the dragon tries to follow both commands simultaneously.