How does August's disability negatively affect his family?

I don't think the challenges are, in the end, negative. Any challenges regarding Augie actually bring the family closer together. Auggie's family is crucial to his growth and maturity over the course of the novel. The other Pullmans are a constant source of support, encouragement, and love, no matter how rough it gets of Auggie as he tries to fit in beyond his family unit. Family is also important because other characters do not have the luxury of a close family like Auggie's: both Justin and Miranda comment that the Pullmans' family dynamic is so special because they themselves come from split-parent families. While the universe has been unkind to Auggie in many ways, in perhaps the most important way it has blessed him with a loving, supportive family that can help him get through almost anything.