I think you're a
Poem
Sometimes I'm ugly, sometimes I'm art,
The quality of poems varies.
Sometimes I'm near and dear to your heart,
Love poems.
I have been known to bring about pain,
Love poems -> confession -> rejection
And yet I deliver relief just the same,
Poems are often used as an emotional outlet.
You know where I live, yes you know it best,
The poems you write come from your head.
For I am not hidden like all of the rest.
The rest of your thoughts remain thoughts.
I come from a family where we each have a twin,
Poems often come in rhymes, so each line has a 'twin'.
Even when we're well nourished, we always stay thin
Words from a poem are no thicker than the page on which they're written.