Sadly, you have no legal right to custody or visitation with your step-children. In order to have any parental rights you would have had to have adopted them, but that would have meant that their mother's parental rights would have been terminated. You might be able to obtain a guardianship over the children if both parents consent.
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None whatsoever if you're talking about custody (living with you after a divorce rather than their father). If you're talking about visiting you as if you were their biological mother and had alternate weekend visits, then that would be the decision of their father. You have no right/standing when it comes to the children.
Neil M. Colman
Mr. Colman is licensed to practice law in Michigan. The response herein is not legal advice and does not create an attorney/client relationship. The response is in the form of legal education and is intended to provide general information about the matter within the question. Oftentimes the question does not include significant and important facts and timelines that, if known, could significantly change the reply and make it unsuitable. Mr. Colman strongly advises the questioner to confer with an attorney in your state in order to ensure proper advice is received.