"Step-brother" is a noun and does not have an opposite.
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The word step-brother in the Spanish language is hermanastro. The Italian word for this is step-fratello and in French it is demi-frere.
Your Step-brother. If he isn't your brother but he is your father's son, he's you.
un frère ? (a brother?)
It means "the mother of your step brother"
There really is not an opposite. What is the opposite of friend? Maybe 'enemy', but not really that, either. What is the opposite of 'une' or 'one'? That is like asking what the opposite of a piece of wood is . . . there is none.