On the sovereignty of God: do prayers change God's will?

[ilovepsalm139]

I'm sixteen and one of seven children. A few weeks ago, we found out that my fourteen-year-old sister had a brain tumor, which was then successfully removed. However, it was cancerous and she has to undergo treatment (chemotherapy, etc) soon. Obviously I've been really upset.
Lately I've been thinking a lot about the different outlooks on free will and God's sovereignty. For the first time ever, Calvinist ideas have made at least some sense to me. This isn't exactly the question I have, but it's probably some good back round info.
Ever since this happened, I've been praying a lot. We have so many friends praying for us, and people keep reminding me how powerful prayer is. Despite this, I've had so many doubts. If I don't pray enough, will God not save my sister? Do my prayers really change God's mind/ will? I then thought of the story of Jonah "When God saw by their actions how they turned from their evil way, he repented of the evil he had threatened to do to them; he did not carry it out." (Jonah 3:10). But does God create evil? Evil is the absence of good, and is so opposite of God, the one, the true, the good, the beautiful. Does God send suffering? My sister said that she thinks that God sent this because he knew my sister and our family could handle it, and that this is a way of his bringing us to him. And it's true, so many people now are praying who have never prayed before, and I'm touched.
But I really just don't know what to think. I've always thought that terrible things simply happen because of the fall, and God gives us the grace to cope with it. Now I just don't understand: does God cause evil? Does he make children suffer in order to win people to him? Do my prayers even matter? Does his mercy depend on what we "pay" him in prayers and sacrifices?
All this time I've felt so angry at God and I've wanted to turn away from him, but I can't seem to do it. I think that it is him bringing me to him, but I don't know how to put my trust in him, because I am afraid he will take my sister.

[Fr. Vincent Serpa]

Hi,

You are a good, thoughtful person! Your fear of not praying enough sounnds a bit obsessive compulsive. Relax' God loves your sister more than you and your family do--as is evidenced by any crucifix that catches your eye.

God does allow evil that a greater good may come of it. The writers of the Old Testament were writing for simple people. So they attribute human emotions to the Father that He doesn't really have. He isn't swayed by our behavior. But He does challenge us. And prayer IS powerful--because it changes US. Those who have been praying for your sister have been benefiting from their love for her and their own unselfishness. God has already determined what He will do for your sister. Her illness may will be for the benefit of your family and all those who are praying.

When we ask Him for something, we need to always be willing to accept not getting our way. He needs to matter more to us than what we are asking for. He is not a big vending machine in the sky. He is the God who suffered and died on our behalf because He loves us so much. He alone knows what is best in every situation. He wants you and your family to come closer to Him and to love Him back

The best part is that whatever happens, it will ultimately be the very best. I will be praying before the tabernacle at 2:30 A.M. for you sister and your whole family--including you.

Fr. Vincent Serpa, O.P.