Yahoo: The Sad Case of The Youngest Person Ever to Be Diagnosed With Alzheimer's The Sad Case of The Youngest Person Ever to Be Diagnosed With Alzheimer's MSN: Teen boy, 19, believed to be youngest person to ever be diagnosed with dementia Teen boy, 19, believed to be youngest person to ever be diagnosed with dementia His sister is his youngest sibling, but his younger sister. Since she is the lone female child, there is no need for the superlative form. Suppose, however, that there are more siblings, and their birth order goes like this: boy, girl, boy, boy, girl.
An only child is both the youngest and the oldest, sure. But you cannot say "the youngest of the two children", now can you? You would never ask someone which of their parents was the oldest. It just doesn’t feel right.
youngest person to ever live, The older parent is older than the other, since you can’t say that the oldest parent (of the two) is *oldest than the other. If you only have one sister and she is younger than you, she is your younger/little/baby sister. If you have several sisters, it gets more complicated: The sisters closest to you in age, but younger, are your younger/little sisters. The youngest sister is then your "baby sister". (US)
