Through marriage your step - sister's daughter is your step-niece!
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If a man has a daughter from a revious marraige, that daughter is the step-daughter of the second wife, and the second wife is the step-mother of the daughter.
3rd cousin I think:-)
Mother's grandson as regards her daughter is either her (daughter's) son or her (daughter's) nephew.
There is no relation that is properly called a step cousin.
Your great grandparents brothers and sisters are your great uncles and great aunts.