Here is a sample prayer for a sister that you can use as a guide:
Dear God,
I lift up my sister to you today. Please be with her and bless her in all that she does.
I pray that you would give her your strength and comfort as she faces the challenges in her life. Please give her wisdom and guidance as she makes decisions, and help her to know that she is never alone.
May she experience your love and grace in a deep and personal way. Please keep her safe and protect her always.
In your holy name I pray, Amen.