Help me find the words? Please

I'm in my mid 40's desperately TTC for 3 years. My niece (not blood but by marriage) offered to give me one of her eggs. I'm so very amazed and grateful at her offer and even though we are still thinking about it I want to write to her to express just what this selfless gesture/gift of a suggestion meant to me. I try to write it but I just can't convey the depth of my gratitude. Anyone able to help me with wording? It probably sounds like a silly request but it means a lot to me. Thank you!