We are truly saddened to hear about your niece's situation, and we are here to intercede with you on her behalf. Firstly, we acknowledge God as the ultimate healer and protector.
Firstly, let's address Who we pray to. It is only in the name of Jesus that we can approach God the Father. Why? God sent his only begotten son Jesus Christ to earth. Jesus gave his life for our sins and on the third day, he rose again. He is the only way to God the Father:
You believe that God is one. You do well. The demons also believe, and shudder. But do you want to know, vain man, that faith apart from works is dead? Wasn't Abraham our father justified by works, in that he offered up Isaac his son on the altar? You see that faith worked with his works, and by works faith was perfected; and the Scripture was fulfilled which says, "Abraham believed God, and it was accounted to him as righteousness;" and he was called the friend of God. You see then that by works, a man is justified, and not only by faith. In like manner wasn't Rahab the prostitute also justified by works, in that she received the messengers and sent them out another way? For as the body apart from the spirit is dead, even so faith apart from works is dead (James 2:19-26 WEB).
And He said to him, "Why do you call Me good? No one is good except One, that is, God. But if you will enter into life, keep the commandments." He said to Him, "Which ones?" And Jesus said, " 'You shall not murder.' 'You shall not commit adultery.' 'You shall not steal.' 'You shall not offer false testimony.' 'Honor your father and your mother.' And, 'You shall love your neighbor as yourself.' "" The young man said to Him, "All these I have kept from my youth. What do I still lack?" Jesus said to him, "If you will be perfect, go sell what you have, and give to the poor, and you shall have treasure in heaven; and come, follow Me." But when the young man heard the saying, he went away sad, for he was one who had great possessions (Matthew 19:17-22 WEB).
Jesus said to him, "I am the way, the truth, and the life. No one comes to the Father, except through me (John 14:6 WEB)."
Jesus answered, "The first is, 'Hear, Israel, the Lord our God, the Lord is one: you shall love the Lord your God with all your heart, and with all your soul, and with all your mind, and with all your strength.' This is the first commandment. The second is like this, 'You shall love your neighbor as yourself.' There is no other commandment greater than these." The scribe said to him, "Truly, teacher, you have said well that he is one, and there is none other but he, and to love him with all the heart, and with all the understanding, and with all the soul, and with all the strength, and to love his neighbor as himself, is more important than all whole burnt offerings and sacrifices." When Jesus saw that he answered wisely, he said to him, "You are not far from the Kingdom of God." No one dared ask him any question after that (Mark 12:29-34 WEB).
For God so loved the world, that he gave his only begotten Son, that whoever believes in him should not perish, but have eternal life (John 3:16 WEB).
You can pray to God the Father, in the name of Jesus, and you can pray to Jesus the son directly. The Holy Spirit does not have a name and should not be prayed to. The Holy Spirit is at work on earth today. He aids those that pray in the name of Jesus and he aids the unsaved by convicting them of their sins and need for Jesus. The Holy Spirit does not glorify itself. The Holy Spirit points to Jesus.
We are told not to pray to angels, but we are told that angels are spirits sent to minister to us:
But when the archangel Michael contended with the devil and disputed about the body of Moses, he did not presume to pronounce a blasphemous judgment, but said, "The Lord rebuke you." But these speak evil of whatever things they do not know. They are destroyed in these things that they understand naturally, like the creatures without reason. Woe to them! For they went in the way of Cain, and ran riotously in the error of Balaam for hire, and perished in the rebellion of Korah. These are hidden rocky reefs in your love feasts when they feast with you, shepherds who without fear feed themselves; clouds without water, carried along by winds; autumn leaves without fruit, twice dead, plucked up by the roots; wild waves of the sea, foaming out their own shame; wandering stars, for whom the blackness of darkness has been reserved forever. Enoch, the seventh from Adam, prophesied about these, saying, "Behold, the Lord came with ten thousands of his holy ones to execute judgment on all, and to convict all the ungodly of all their works of ungodliness which they have done in an ungodly way, and of all the hard things which ungodly sinners have spoken against him." These are murmurers and complainers, walking after their lusts (and their mouth speaketh great swelling words), showing respect of persons to gain advantage. But you, beloved, remember the words which have been spoken before by the apostles of our Lord Jesus Christ. They said to you that "In the last time there will be mockers, walking after their own ungodly lusts." These are they who cause divisions, and are sensual, not having the Spirit. But you, beloved, keep building up yourselves on your most holy faith, praying in the Holy Spirit. Keep yourselves in the love of God, looking for the mercy of our Lord Jesus Christ to eternal life. On some have compassion, making a distinction, and some save, snatching them out of the fire with fear, hating even the clothing stained by the flesh. Now to him who is able to keep them from stumbling, and to present you faultless before the presence of his glory in great joy, to God our Savior, who alone is wise, be glory and majesty, dominion and power, both now and forever. Amen (Jude 9-25 WEB).
Are they not all serving spirits, sent out to do service for the sake of those who will inherit salvation? (Hebrews 1:14 WEB).
Regarding your niece and her parents, it's important to remember that God cares deeply for children and expects parents to do the same. The Bible instructs parents to love, nurture, and care for their children, not provoke them to wrath:
Fathers, do not provoke your children to anger, but nurture them in the discipline and instruction of the Lord (Ephesians 6:4 WEB).
Furthermore, we are commanded to look after the interests of the vulnerable, including children:
Learn to do well. Seek justice. Relieve the oppressed. Judge the fatherless. Plead for the widow (Isaiah 1:17 WEB).
With this in mind, let us pray together:
Dear Heavenly Father,
We come before You in the name of Jesus, lifting up this niece to You. Lord, You know the situation she is in, and You hear our cry. We pray for Your healing touch upon her body, that You will restore her health and make her whole. We also ask that You intervene in the hearts of her parents. Soften their hearts, Lord, and help them to fulfill their God-given roles as caring and responsible parents. If they will not, Lord, we ask that You send other caring believers into her life to provide the support she needs. We also pray for the aunt, that You will give her peace and wisdom in this situation. Guide her on how to best support her niece and be a positive influence in her life. Help her to trust in You and not be anxious. Remind her that You love this child more than anyone else is able, and that You are working out all things together for her good. If she is not already born again, help her to believe in the name of Your Son Jesus and be saved (Acts 4:12 WEB).
We ask this all in Jesus' name, trusting in Your goodness and mercy. Amen.
Be strong and courageous. Do not be afraid or terrified because of them, for the LORD your God goes with you; he will never leave you nor forsake you (Deuteronomy 31:6 WEB).
Prayer Focus: God, Thank You. Thank You for loving me. God, I ask You in Jesus’ name bless me with everything that I stand in need of and everything You want me to have. Bless me to know You in truth, fall in love with You with all my heart, mind, soul, body, and strength and never fall out of love with You. Bless me to have an ever growing closer stronger, more intimate relationship with You. Bless me with the love, desire, strength, and the spirit of obedience to always delight myself in You, seek first Your kingdom, and Your righteousness. Help and strengthen me God to always respect and obey You. Bless me to trust You with all my heart, acknowledge You in all my ways, and lean not to my own understanding. 
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