I've checked the recent Michigan case law and it's not settled as to what they allow and what they don't. So I suggest that if you want visitation with your grandson/nephew, that you bring this case in the local Family Division court. You will need to show the court that while you are the aunt, you have been a "de facto" grandparent to this child since his birth. That means that you've been like a mother to your sister and a grandparent to your nephew.
A de facto custodian is a person who has been the primary caregiver for and financial supporter of a child who has resided with that person for at least 6 months if the child is less than 3 years old or at least one year if the child is older than 3 years old. You may not be the de facto custodian now, but perhaps at one time you were? You'll need to tell the court if the child lived with you, what your relationship is with the child, what your relationship was with your sister, what your sister asked you to do, etc.
If you would like me to find family lawyers in your area, I can do that for you as a free service. I do that for my customers here to point them, and you, in the right direction in their home areas. We can't make recommendations but I can give you a list of family lawyers and a complete, short guide as to how to pick one.
To recap, many states are clear on grandparent rights and third-party rights. Michigan is not so clear-cut. Therefore, if you want to test the waters, by all means bring a case in the local family court if you cannot get dad to comply. If it turns out dad is unfit, you may end up applying for custody. Just keep that notion in mind -- just in case.
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