"In America" begins with Christy asking her little brother Frankie to grant her a wish, and it ends with the same thing. Christy's wishes are an important part of the film whether you believe that these wishes cause things to happen within the character's lives or not.
Christy's first wish is to get her family over the border into America. When the family comes to the border and is speaking with the patrolman, they say they are there on holiday, but Ariel blurts out "DADDY DOESN'T HAVE A JOB!" which causes the whole family to instantly tense. Christy makes her wish on Frankie, and the family is allowed into the country without any hassle.
Christy's second wish takes place at the carnival when Johnny is trying to win an E.T. doll for Ariel. The game requires that you double down on your money for each extra throw, and Johnny determined to not let his children see him fail, reaches a point where they have to keep buying throws because they cannot afford to lose the money.
When Johnny reaches his final throw, with all their rent money on the line, Christy makes her wish. She comments that her father always continued to believe it was him that won the E.T. doll.
Christy's final wish, and perhaps the most valuable of all was originally being held for her mother's labor. Christy is carefully planning the wish, as her mother will lose the baby if it comes too early and her own life if it comes too late. Christy's mother goes into labor early, but both the baby and Sarah survive without Christy's wish. What Christy does for the baby instead is to be her donor for a blood transfusion.
"Say goodbye to Frankie dad. Say it," two of the best lines in the movie, but the thing is the dad doesn't do it on his own, but with Christy's final wish. Christy wishes for Frankie to let them let go. Their family will never be what it was, but they can't hold on to the guilt about Frankie's death forever. I think this is the most powerful wish Christy could have made.
Christy's wishes are never solely for her own benefit, but instead in the interest of her family, which shows how much they mean to her. The fact she never wastes a wish also shows how wise she is for her age. I think that within the film, Christy's wishes really do have an impact on the course of events. Everything she wishes for happens, and whether it's Frankie looking out for her, or simply her willpower to see these things happen, it's all linked.