Mary is a widow. She has a son, Martin.
Peter is a widower. He has a son, Paul.
Mary and Peter get married. Martin and Paul are step-brothers. Martin is Peter's stepson. Paul is Mary's stepson.
Mary and Peter have a little girl, Maribelle. Martin and Paul are both her half-brothers. She is their half-sister. They share one parent.
I have heard "full sister" but not "blood sister." That may be used elsewhere, though.
Be careful with the word "real" -- A person's half-sisters/half-brothers are very real. To use "real" for some relationships suggests others are not important. If you want to see a mother who has adopted a child get angry very quickly, ask her if she has any "real" children of her own. My coworker has four half-siblings. She would be irate if you suggested that only her sister from her parents' first marriage is her "real sister."
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