I agree with my colleagues and I suspect that Attorney Davis has nailed it on the head with his analysis. If the assets were jointly owned between spouses, then there is nothing TO probate, and nothing controlled by your aunt's Will. If that is the case, the Will is still supposed to be filed with the probate court, but it would never be admitted to probate. That sounds to me like what has happened.
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Most WA attorneys will not be familiar with Michigan law. You may want to ask your question again listing a city in Michigan. The biggest problem your mother is likely to face is finding a copy of the will. Her brother-in-law has little reason to produce it, or to have kept it. Much of your Aunt's property may also have passed by beneficiary designation, not by her will. A Michigan probate attorney can help your mom determine if this is worth pursuing.
This answer provides general legal information and should not be construed as legal advice to be applied to any specific factual situation. It is not intended to create and does not create an attorney-client relationship. The attorney writing this post is licensed in Texas and Washington only and the laws of your jurisdiction may differ.
Many married couples own their property as joint tenants with rights of survivorship. In such cases when one spouse dies, there is no probate because there is no estate. All of the property belongs to the survivor who merely has to file paperwork to remove the deceased spouses name. The easiest way to check this out if your aunt owned a house may be to contact the county clerk/recorder in the county where the property is located and check the register record on the title to see the deed and whether an affidavit of survivorship was filed.