Forgive me for starting to debate.
You are right. Your sister is in bondage and needs deliverance. If she was a Christian and has turned to witchcraft, her Christianity was one of form only, not truth.
2Ti 3:5 Having a form of godliness, but denying the power thereof: from such turn away.
God does not change people's will. What we can pray for is for God to work circumstances to bring her to rock bottom so she will realize her need for salvation.
Lord, you know her heart and understand why she has turned away. I pray that in the process of bringing her down that you will protect her so that she suffers no bodily harm. Work circumstances so that she sees her need for salvation. Bring to mind all the good she knew about Christianity, even if she never was truly a Christian in the first place.
Father, I also ask that you give Kassa the wisdom to know how to both pray for her sister and to speak with her sister. I pray her sister will experience the love of Christ flowing through Kassa as they talk.
We commit her sister to your charge. You are the Good Shepherd who laid down your life for her. Help her to see this. I ask in the name of Jesus.