We are truly sorry to hear about your dad's health condition, and we are here to support you in prayer and counsel. Let's first turn to the Scriptures for comfort and guidance. The Bible tells us, "Therefore, confess your sins to one another, and pray for one another, that you may be healed. The insistent prayer of a righteous person is powerfully effective" James 5:16 WEB.
Healing is always God's will for us, as we see in 3 John 1:2 WEB, "Beloved, I pray that you may prosper in all things and be healthy, even as your soul prospers." We stand with you in faith, believing that God can and will heal your dad completely. Let's pray together:
Heavenly Father, we come before You in Jesus' name, lifting up our sister and her dad. We believe that You are the same yesterday, today, and forever, and that Your healing power is still available to us today. We rebuke any spirit of infirmity attacking his lungs, and we command them to be made whole and restored in Jesus' name. We pray for wisdom and discernment for the doctors, that they may provide the best treatment possible.
Lord, we also ask for Your guidance regarding how long to stay in town. We trust that You will make it clear when the time is right to leave, and we ask for peace and assurance that that decision is from You. We pray for the right words to encourage him, saying "Let no corrupt speech proceed out of your mouth, but such as is good for building up as the need may be, that it may give grace to those who hear" Ephesians 4:29 WEB.
Father, we also pray for joy in this time. Though it may be difficult, we know that Your joy is our strength, and that You are always with us. Comfort us and show us Your everlasting loving kindness and faithfulness. Help us to communicate well with each other and to be loving and kind to one another, understanding that love covers over a multitude of sins 1 Peter 4:8 WEB.
Thank You, Lord, for hearing our prayers. We trust in Your goodness and Your power to heal and restore. In Jesus' name, we pray. Amen.
As for encouragement, remind your dad that God is always with him and loves him deeply. Share with him the many promises of God's goodness and faithfulness, such as Psalm 23:4 WEB, "Even though I walk through the valley of the shadow of death, I will fear no evil, for you are with me. Your rod and your staff, they comfort me."
Lastly, be sure to include Jesus in all prayers. It is only in His name that we have access to God. Sending prayers to God in any other name will be unsuccessful.
Prayer Focus: God, Thank You for loving me. Thank You for loving me, Jesus. 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, soul, mind, 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. 
