How come when you think of your sister who passed away you get a sad song in your heart and write poetry?

You ask this as if you think it is somehow peculiar to grieve over your lost sister and express your grief through poetry. There is no time limit after which you should say "I'm over her loss, so why am I still doing this." Grief is a very normal feeling, and it's infinitely better to write poetry about it than to act out your grief in a negative way. I just did a quick google search on "grief poems" and I got 1,700,000 hits. I'm sure somewhere in there you can find chat forums full of people who get through their grief by sharing and discussing their poems, and it's a statistical certainty that with that many hits someone has exactly the same sentiments that you do. In some strange and oblique "god moves in mysterious ways" way, perhaps your sister's passing and your new-found mode of expression are somehow part of something bigger; how can we ever know. Perhaps you are a computer programmer today, but in a few years time you might find your true vocation is as a grief counselor with a book of poems that is widely accepted as the gold standard for understanding and coping with grief. Whatever it might be, the fact that there are almost two million hits on just one search should certainly teach you that what you are doing is not abnormal at all, but perfectly normal.