We understand your concerns and are here to support you in prayer and counsel. Let's first address the issue of your niece. It's wonderful that you are interceding for her to return to prayer.
If you make my word your home, you will indeed be my disciples. You will know the truth and the truth will make you free. John 8:31-32 WEB.
We pray that your niece will find her way back to God's word, that she will seek Him in prayer and find the truth that sets her free. May God renew her love for Him and for her family. She is probably going through a phase in life, and we pray that God help her navigate through it and return to her previous ways.
We see the peace of mind you want for your brother, but we must ask. How is his personal relationship with the God of the Bible? There should be a personal relationship with Jesus.
Pilate said to them, "You have a guard. Go, make it as secure as you can." So they went with the guard and made the tomb secure. Now after the Sabbath, as it began to dawn on the first day of the week, Mary Magdalene and the other Mary came to see the tomb. Behold, there was a great earthquake, for an angel of the Lord descended from the sky, and came and rolled away the stone from the door, and sat on it. His appearance was like lightning, and his clothing white as snow. For fear of him, the guards shook, and became like dead men. The angel answered the women, "Don't be afraid, for I know that you seek Jesus, who has been crucified. He is not here, for he has risen, just like he said. Come, see the place where the Lord was lying. Go quickly and tell his disciples, 'He has risen from the dead, and behold, he goes before you into Galilee; there you will see him.' Behold, I have told you." Matthew 27:65-28:7 WEB.
We pray that your brother will find peace in the resurrection of Jesus. Only through Jesus can he have the kind of peace that is beyond human understanding.
Do not be anxious about anything, but in everything by prayer and petition with thanksgiving let our requests be made known to God. And the peace of God, which surpasses all understanding, will guard our hearts and our thoughts in Christ Jesus. Philippians 4:6-7 WEB.
As for your request for good health, we pray that God will grant you and your family good health, but remember that God is more interested in our spiritual health than our physical health. We pray for both.
Beloved, I pray that you may prosper in all things and be healthy, even as your soul prospers. 3 John v2 WEB.
Let's also remind you that God's word is clear about how we should approach Him. It's only through the name of Jesus that we can come to God. We should always make our requests known to God in the name of Jesus.