If there is a death of the custodial parent and how does one obtain custody as the biological mother?

I am the non-custodial parent of my 11 year old son. His father passed away 3 days ago and he is with his grandmother. The father had cancer and moved back to his mothers with our son when he became too sick to work about 3 weeks ago. He never told me he was back in Michigan and his mother never contacted me to let me know he passed! They enrolled him into school and I assume the father had designated the grandmother as guardian in a will. My son is still at her house and she is refusing to let him visit upon his request! She wants me to meet with her to sign something that she says will finalize everything. I am becoming concerned that she may be up to something and unless I agree to sign a paper she is going to keep him from me. I don’t know what rights I have right now or whether I could just go get him without her consent. I have my parental rights just not custody. Where does this leave me at this point with my rights to my son? Does she trump me without ever having any court proceedings and don’t I automatically retain custody as the mother after the father’s death? What do I do first? Apply for custody in probate court? How exactly does one just obtain custody after the death of the custodial parent? How do I know if it's legal to insist she let me have him? I have 2 other children in another relationship and am absolutely certain there's no way anyone would find me as unfit in any way. I am physically disabled as a result of a spinal cord injury from a car accident and can easily care for my other children so unless that deems me unfit or possibly the fact that I am collecting SSI and welfare food and insurance benefits as a result of my disability and inability to work, what should I be most concerned about right now? I very much appreciate your input and simply pointing me in the right direction as I am truly lost at this point!