Aww this was beautiful. The rhyme scheme is unusual but worked well. The rhythm was really well done, consistent and easy to read.
I love the repetition of 'forevermore' and 'he hasn't died'...they made constrasts in the poem and contributed to the kind of sad mood. The last line was very good as well, especially the rhyme.
I've no idea who your OC is, as obviously I havent' read the fic. *puts it on her to-do list* :p
They've fallen in love even though they are adopted brother and sister?/ How tragic..
I honestly thought your poem was really good. I don't really have much else to say about it.