Well, this is still a rather precocious child who would use the word, "sibling". But, maybe she's just smart and remembers what the doctor called she and her brothers. The story you tell
happens all too often. The speaker is "believable" through your character's speech pattern and use. It is sad that the story "is" believable. I will not comment on your use of "free verse" as I am not practiced in the form.