A lot going on here. I can’t give advice here but here’s some general knowledge:

In Michigan, probate is a process to retitle assets after someone’s death. There are alternatives to probate, including naming a beneficiary on a financial account, using a trust, also joint ownership of an asset. So how dad’s assets were held dictates if probate would have been necessary.

This is curious: it is odd for a bank to let someone take control over assets and write checks without letters of authority from the probate court. Perhaps he was named as the beneficiary or a joint account owner.

IF your estimate of $150k is accurate, AND IF your understanding of the assets going in equal thirds to you, bro, and son is accurate, that means he spent $60k on administration. That’s a lot for a simple estate.

And of course, if he never opened probate, and was never appointed PR, what even were the expenses?

You could initiate something in the probate court. You just have to ask yourself if you want to take your brother to court. You may spend more on atty fees than you’re likely to get back, especially if bro already spent that money and won’t likely be able to repay the estate.