Let us first address the concern about your sister's spiritual state. It is commendable that you are concerned for her salvation and desire to pray for her. In the Bible, we are told, For there is one God, and one mediator between God and men, the man Christ Jesus, who gave himself as a ransom for all. This was the testimony given at the proper time. (1 Timothy 2:5-6) WEB
While it is possible for a person to be influenced by demons, it is important to approach this situation with caution and discernment. Demonic possession often manifests in more extreme ways than what you have described. However, demonic oppression or influence can manifest in various ways, including anxiety, irrational behavior, and anger. It is clear that your sister's behavior is causing concern and distress, and it is important to address this with prayer and love.
Regarding the spiritual warfare, let us rebuke together in prayer first.
Heavenly Father, we come before you in the name of Jesus Christ to intercede for our sister's sister. We plead the blood of Jesus over her life and pray that you would set her free from any demonic influence or oppression. In the name of Jesus, we rebuke any spirits of anxiety, anger, and irrationality that may be influencing her. We command them to leave her in Jesus' name. Father, we ask that you would fill her with your peace, love, and joy. We pray that her heart would be open to the truth of the Gospel and that she would come to know Jesus as her Lord and Savior. We declare your promise from that if we confess with our mouth that Jesus is Lord, and believe in our heart that God raised him from the dead, we will be saved. (Romans 10:9) WEB
Now, let us address your sister's behavior and your interactions with her. It is important to approach her with love, patience, and gentleness.
Continuing in prayer, we ask you, Lord, to guide us in how to interact with her. Give us the wisdom to know when to speak and when to remain silent. Help us to show her the love of Christ in our actions and words. We pray for her any blame or anger to be from conviction of the Holy Spirit, we ask that she would be drawn to repentance and faith in Jesus. Lord, we ask you to soften her heart and open her eyes to the truth of your word. We pray for her eyes to be opened and she will know how wide and long and high and deep is the love of Christ. (Ephesians 3:18) WEB
Regarding her comments about sola scriptura and pastors. Sola Scriptura is the theological doctrine that the Christian Scriptures are the sole infallible rule of faith and practice. We reject her fault finding. We affirm, All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness. (2 Timothy 3:16) WEB
Regarding her comment on where she will go after death. This is a concern, as it indicates that she may not have a personal relationship with Jesus Christ and we must continue to pray for her salvation. We affirm, For God so loved the world, that he gave his one and only Son, that whoever believes in him should not perish, but have eternal life. (John 3:16 WEB). We affirm she will spend eternity separated from God if she does not repent and put faith in Jesus Christ. You have said it well, Jesus is the only way to God.
In closing, continue to pray for your sister and trust that God is at work in her life. Remember, that our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. (Ephesians 6:12) WEB
Remember and be encouraged, be strong, stand firm in your faith. The God of love and peace will be with you. (2Corinthians 13:11) She may continue to reject your prayers and concern, but continue to trust God's timing to bring her to salvation.
thank you 
