I am so sorry that you and your family are going through these troubled times. It is quite sad when people impose their views, ideas, or beliefs on families, especially when troubles like this occur. One problem that is happening here is the truth taken from the Bible has been mixed with false beliefs. As for what the Bible teaches, I know you will be comforted to see how God, through Jesus is taking care of things.
There is a wonderful, free, Bible study aid that answers questions to problems we face and leads us to make wise decisions. All the chapters are important to read. Chapter 7 answers your questions.
As I said, this is a Bible study aid. The Bible, which is inspired of God and beneficial for teaching, has the answers. (2 Timothy 3:16) Also remember that God is comforting us through his scriptures.
2 Corinthians 1:3, 4: "Blessed be the God and Father of our Lord Jesus Christ, the Father of tender mercies and the God of all comfort, who comforts us in all our tribulation."
Feel free to contact Jehovah's Witnesses for any questions you may have. They will take you to your Bible for answers. Among the many scriptures, a very important one that all should heed is found at 1 Thessalonians 4:11 that says...."mind your own business". This is the time that family needs to make decisions because when all is said and done, you and your parents have to live with the consequences.
Even if you are unable to make decisions for your parents or your brother, know that your prayers will not go unanswered when you pray for wisdom from the true God, Jehovah. (Psalm 83:18)
I hope this helps and that you can find some peace during these times.
First: God will do what is best for your brother's soul. This might be difficult for you to see because you love and miss him. So, God CAN bring him back, but will only do so if it is best for his soul.
There is nothing wrong with signing a DNR. If God wants your brother to live, he will live. If your brother will go to heaven if he dies, but will succumb to temptation and become damned if he lives to long, God will protect your brother's soul, and he will die no matter what doctors do - even if they keep his lifeless empty husk functioning without his mind and soul. Refusal to sign a DNR makes money for the hospital, drains your family's finances, may increase your brother's suffering, and will make you dependent upon charity, but It does not effect your brother's soul and will not extend his life. If anything refusing to sign a DNR demonstrates a lack of faith in God.
I do not know why your congregation is being so cruel to you. It might be that they have taken a "Pro-Life" political stance that they cannot justify through Scripture and so are adopting other extremist dogmas to cover their lack of biblical support.
God never forbid organ donation or transplant. People who have never read a single word of the original Hebrew text of the Commandments have made that up based on what they hope the text might say, and a desire to feel superior to other people (mostly the latter).
Finally: Don't blame God for what ignorant, selfish people do even when they claim to be Christians.
Do not loose faith in God; just find a new church or synagogue.