_Blench_ signifies to flinch.

If 'blanch' be the word, the next ought to be '_hair_.' You cannot here use _brow_ for the _hair_ upon it, because a white brow or forehead is a beautiful characteristic of youth.

'Sickly ardor o'er' was at first reading to me unintelligible.

I took 'sickly' to be an adjective joined with 'ardor,' whereas you mean it as a portion of a verb, from Shakspeare, 'Sicklied o'er with the pale cast of thought.' But the separation of the parts or decomposition of the word, as here done, is not to be endured.
Let me now come to your sister's verses, for which I thank you.

They are surprisingly vigorous for a female pen, but occasionally too rugged, and especially for such a subject; they have also the same faults in expression as your own, but not, I think, in quite an equal degree.