If I were the autistic sister, I would acknowledge the sacrifice of my elder sister. However, I would also be quick to tell her not to use my autism as her reason for not being able to find a husband.
I might look cute at the age of 18, and she might still have the power of youth to resist the onslaught of loneliness. When I am 28, and my sister 37, she might think differently. If I'm no longer cute, what remains is a burden - an old burden. And she might blame me for her not getting married. At 37, it would be even more difficult for her to find someone to marry her, even if she was prepared to drop her "buy one get one free" condition.
If I were the autistic sister, I do not want to be the reason for my sister remaining a spinster, and worse still, the excuse. At the very least my sister should not announce to the media I would be the reason for her spinsterhood. Even without my condition, there could be many reasons why she could not get married.