There are a number of different considerations, here, and you either have not posted or do not know enough information to determine if what your mother is telling you is true and/or whether or not you have any basis for making a claim.
You are not an heir, at least, not as that term is defined under Michigan law, because your mother survived you, as well as apparently your aunt and uncle. The three of them (and any other siblings, or surviving children of deceased siblings) would be heirs. Heirs, in this sense, means those people who would be in line to inherit property, in the absence of any Will or Trust.
What it SOUNDS like may have happened in this case is that your grandmother may have transferred title to all of her assets to your mother, either by joint ownership, beneficiary designation or in some other manner. If that was not the case, then your mother would have no way of denying her siblings their share of the assets.
Since this was apparently changed shortly before your grandmother's death, you would need to know if your grandmother was of sound mind when the changes were made, or if she might have been subject to undue influence by your mother. This is not easy to prove, and there would need to be some strong evidence, such as medical records, testimony of witnesses, and the like, in order to overcome the presumption that everything was handled correctly. It would be even more difficult if the estate planning was done through an attorney, who would presumably testify against you, and state that this was all done based on your grandmother's intent.
Without knowing more, there is no way to proceed with any confidence. Your mother's comments about creditors and mortgages may be red herrings, but they certainly do not make a lot of sense, without knowing more about the context. It is unclear how the title to the assets was/is held. You may be able to determine this, based on public records, depending on the nature of the assets involved. (You can search online records in relation to real estate, for example.) You may also be able to get some information from your grandmother's attorney, if she had one, but he or she may be unwilling to share much, particularly if your mother has hired the attorney to represent her, with regard to administering whatever needs to be administered of your grandmother's estate, trust or otherwise.
You have a tricky situation and your mother would appear to be holding most of the good cards, at the moment.
I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in the subject area in your state. The law changes frequently and varies from state to state.
Mr. Frederick has given you good advice.
You mention that the estate is open. Have you reviewed the file in the probate court? Can you ask your mother about seeing the will or trust?
A lawyer may be able to get more information than you can because of the knowledge and persistence of the lawyer, so you may want to hire one to assist you.
I am licensed to practice law in Michigan and Virginia. My office is in Lapeer. You should not rely on this answer. You should consult a lawyer so you can tell the lawyer the entire situation and get legal advice that is precisely tailored to your case.
Q: How do I find out if I am an heir?
A: Go to the probate court for the county in which your GM died and find out if an estate has been opened.
Q: "I believe my grandmother would never exclude her grandchildren"
A: My experience is that grandparents seldom include bequests to grandchildren
Q: "she also stated to me she has control, and could f@@k her brother and sister as she wants"
A: I hope you take after your father.
Q: "And she says she can not report the death or close the estate for a year, due to the fact that my grandmother had many mortgages in her name for family, so has to wait "
A: Your mother has been watching too much daytime TV.
Get a real lawyer who can get to the bottom of this.