youngest person to have a heart attack - HEALTHY
The "again" means at another time or once more as in, for example: I shall not look upon his like again — Shakespeare Today is the oldest you've ever been and the youngest you'll ever be again. is a literary quote by Anonymous that has been around for a long time, and I think that that is proof that the again is necessary. For example, you have not heard anyone quote, you'll never be younger ...
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There is no specific name for that particular sibling which would make it clear to the listener that you have two younger sisters and you're referring to the elder of the two. There are two points of age-comparison: she's younger than you but older than the youngest. 'Middle sister' could be taken to mean that you have three sisters. My youngest brother works in the back while my elder, younger brother takes orders at the counter." "The older of my brothers." Comes across as a grammatically awkward statement.
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It doesn't sound right when I attempt to annunciate it. I could be wrong, since I don't have a PHD and am just an ESL teacher, but it doesn't sound smooth.