Under Attack

I am struggling right now. So much is going on inside my head and it causes my heart to hurt. I have a sister-in-law with Stage 4 Bone Cancer. She is not doing well at all. I am so proud of her that she has come to the Lord and her faith is strong. That brings such joy to me. But I have another sister-in-law who first of all does not see this conversion as either something genuine or even possible. She claims to know their is a God. I can not say she is an atheist but she does not believe in what he can do and she is one that when you try to speak of the Lord she makes snide remarks and makes you feel like you are believing in a fairy tale or something if you think he can heal such a strong disease like the Stage 4 cancer. I know God can do all things. In my life I have experienced such miracles myself. But the battle is hard with her. I can not say she is winning because I still hold firm to my beliefs but the battle is tiring. I vowed to the Lord many years ago to never let anyone enter or leave my life without me witnessing to them the true power and love of God but now I feel like I am being tested. I care deeply for my sisters-in-law. One is dying and even though that is sad because she will leave behind 3 children and 6 grandchildren she has accepted Jesus into her heart and will be in a much better place if the Lord calls her home and for that I am grateful. My other sister-in-law is the only close (proximity-wise) family that we have here with us and to hear her comments I feel my faith is in constant attack around her and it is wearing me out. I pray and I pray. I want to witness to her and a part of me wants to just make her see how wrong she is and how if you believe there is a God why can't you accept he can do anything but so far it is not working. How do you handle such difficulties? How do you maintain strong when you feel under attack? How do you cope?