This could be based on pure opinion.
it was a love letter for his brother
a sixteen line poem is a sixteen line poem, not a sonnet, consisting of fourteen lines.
A sonnet is a poem that consists of fourteen lines of iambic pentameter.
I am a sonnet, apparently.
Sonnet 141, written by William Shakespeare
A sonnet is an example of a poem.
14 lines in a sonnet
Sonnet
Alfred Lord Tennyson wrote a poem entitled "The Kraken" it is fifteen lines and similar in rhyme scheme to that of an English Sonnet.
A sonnet IS a poem. Not a part in one. If you had just used google you'd of known that immediately.
The line "the darling buds of May" appears in Shakespeare's Sonnet 18, which may be the poem you are looking for.
It's a sonnet of course.