How do I let her know that I don't want to marry him?

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My neighbor has been very kind and generous to me. I always appreciate her everyday and she is aware I am grateful. She likes me a lot. I don't know why. Recently she told me that I'm like her daughter and that she will always see me as her daughter. She is always happy to see me. She trusts me with her house keys and she has currently made me in charge of packing the items she sales and making sure they are safe. Well, Saturday morning there were some women who visited her. I was in the extra house counting the items she was planning to sell. I heard her tell them ," I want this girl to marry my son who is in highschool". I thought to myself that I hope she is not talking about me but then she shouted, " isn't it Mildred, you will marry my son". I was shocked and you couldn't imagine the look and disappointment on my face. Last week she had jokingly told me that she will give me as a wife to his firstborn child but I told her that he was like my younger brother. Then she said she was joking. I told her that I knew she was joking. I assumed it ended there until her Saturday morning confession to the other women. Her second son is in highschool. I am 10 years older than him. He is my younger brother to me. He is in highschool in a neighboring country. Two months ago her second son was in Kenya for the holidays. Every week he kept telling me to be his girlfriend but I insisted that to me he is like my younger brother but he never believed. Frequently I kept praying wondering how I would convince him to see me as his elder sister. So around four weeks ago when he was about to leave and go back to highschool outside Kenya, I got an idea. So I showed him my highschool ID card which thankfully had my year of birth and date of admission. All of a sudden his eyes lit up. He couldn't believe it. He started seeing me like his big sister and completely stopped his relationship requests. He even started advising me and saying that I'm late for marriage and that I should try get a child. I was happy he could finally see me as his sister and he travelled back to school happily. The problem now is how do I tell his mother that I don't want to marry her younger son? I tried but she said age is just a number. I don't know how to make her abort that idea from her mind without it affecting our friendship. What do you think I should tell her? I don't want to be rude. I'm trying to avoid curses. The image is of the chicken I am eating this Sunday night. I bought it for only half a dollar. It is fried and tasty.