Glad that your dad has a diagnosis, and that he is communicating with you. I'm so sorry about your sister and I have been praying about that, that her heart will be softened!
It's so awful when our heart's longing is given a "no" or a "not now." Here are a few scriptures that help me when I'm struggling with disappointment (like now, you know what I'm dealing with in my own health situation and I'm crushed in spirit, but I will overcome!). Perhaps they will help you as well.
I know not everyone is the same faith or religion and some might not believe, so I really hope I don't offend anyone by sharing this.
I [God] know what I’m doing. I have it all planned out—plans to take care of you, not abandon you, plans to give you the future you hope for. ~ Jeremiah 29:11
If your heart is broken, you’ll find God right there; if you’re kicked in the gut, he’ll help you catch your breath. ~ Psalm 34:18
Meanwhile, the moment we get tired in the waiting, God’s Spirit is right alongside helping us along. If we don’t know how or what to pray, it doesn’t matter. He does our praying in and for us, making prayer out of our wordless sighs, our aching groans. He knows us far better than we know ourselves, knows our pregnant condition, and keeps us present before God. That’s why we can be so sure that every detail in our lives of love for God is worked into something good. ~ Romans 8:28
Pile your troubles on God’s shoulders—he’ll carry your load, he’ll help you out. He’ll never let good people topple into ruin. ~ Psalm 55:22
Don’t fret or worry. Instead of worrying, pray. Let petitions and praises shape your worries into prayers, letting God know your concerns. Before you know it, a sense of God’s wholeness, everything coming together for good, will come and settle you down. It’s wonderful what happens when Christ displaces worry at the center of your life. ~ Philippians 4:6-7
Stay with God! Take heart. Don’t quit. I’ll say it again: Stay with God. ~ Psalm 27:14
“God has lost track of me. He doesn’t care what happens to me”? Don’t you know anything? Haven’t you been listening? God doesn’t come and go. God lasts. He’s Creator of all you can see or imagine. He doesn’t get tired out, doesn’t pause to catch his breath. And he knows everything, inside and out. He energizes those who get tired, gives fresh strength to dropouts. For even young people tire and drop out, young folk in their prime stumble and fall. But those who wait upon God get fresh strength. They spread their wings and soar like eagles, They run and don’t get tired, they walk and don’t lag behind. ~ Isaiah 40:28-31
Whatever I have, wherever I am, I can make it through anything in the One who makes me who I am. ~ Philippians 4:13