We are truly saddened to hear about your niece's situation, but we are thankful that you have reached out to us for prayer and support. Firstly, we want to remind you that God loves your niece dearly and has not forgotten her. As it says in Psalm 68:5-6, "A father of the fatherless, and a defender of the widows, is God in his holy habitation. God sets the lonely in families; he brings out those who are bound with chains; but the rebellious dwell in a dry land."
Your niece's diagnosis of borderline personality disorder may be challenging, but we know that with God, all things are possible (Matthew 19:26). We pray that God will begin a work of emotional healing in her life. May He comfort her heart and mend the broken pieces caused by the lack of parental love. As it says in Psalm 34:18, "Yahweh is near to those who have a broken heart, and saves those who have a crushed spirit."
We pray for her protection in the psychiatric hospital. May God's angels encamp around her, keeping her safe from all harm and evil (Psalm 34:7). We also ask that God will grant the doctors and staff wisdom and compassion as they care for her (1 Kings 3:9).
We agree with you in asking God to bring her home soon. May He work in the hearts of the medical professionals to guide them in their decisions regarding her care. Trust that God is in control and that His timing is perfect. As you wait, be encouraged by the words of Lamentations 3:26, "It is good that a man should hope and quietly wait for the salvation of Yahweh." Also, Romans 8:25, "But if we hope for that which we do not see, we wait for it with patience."
Lastly, we want to remind you of Jesus Christ and the power of the Holy Spirit. It is only the name of Jesus that we have access to God. Also, there is no other name by which we can be saved.
Let us pray together:
Dear Heavenly Father,
We come before You today, lifting up this dear young lady who is struggling with borderline personality disorder. We ask, Lord, that You would begin a work of healing in her heart and mind. Comfort her, Father, and show her the love that she has been missing. Protect her while she is in the hospital, and guide the doctors as they treat her. In Your perfect timing, Lord, bring her home to her family. We ask all these things in the powerful name of Jesus Christ, our Lord and Savior. Amen.
We encourage you to continue praying for your niece and trusting in the Lord. Share the love of Jesus with her, and let her know that she is not alone. Remember the words of Deuteronomy 31:6, "Be strong and courageous. Don't be afraid or scared of them; for Yahweh your God himself is who goes with you. He will not fail you nor forsake you."
We also encourage you to share the Gospel of Jesus Christ, the only begotten Son of God, who chose to become a man and die on the cross as the only sacrifice for the sins of the world and rose again on the third day, to your niece. It is only by the name of Jesus that we have access to God and it is only by His name are we saved.
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. 
