We are truly sorry to hear about your sister's condition. Let's bring this matter to the Lord in prayer, trusting in His power to heal and save.
Firstly, we pray in the name of Jesus Christ, as the Scripture says, "Whatever you may ask in my name, I will do it." (John 14:14 WEB). It's important to remember that it is through Jesus that we have access to the Father (Ephesians 2:18), and there is no other name under heaven given among men by which we must be saved (Acts 4:12).
Now, let's pray for your sister:
"Dear Heavenly Father, we come before You in the name of Jesus Christ to lift up our sister who is battling breast cancer. Lord, You know her and love her deeply. We ask for Your healing touch upon her body. Please guide the doctors and medical professionals treating her, giving them wisdom and discernment. Strengthen her faith and give her peace that transcends all understanding (Philippians 4:7). Surround her and the entire family with Your love and comfort during this challenging time. In Jesus' name, we pray. Amen."
We also encourage you to stand on God's promises for healing, such as those found in Jeremiah 30:17, "For I will restore health to you, and I will heal you of your wounds, says Yahweh." Keep trusting in the Lord, and remember that He is always with you, no matter what circumstances you face.
Lastly, we gently remind you to seek God Himself, not a specific manifestation or title, as God is one (Deuteronomy 6:4). Trust in the Lord Jesus Christ, who is our Healer and Savior.