1. The form 4868 is an extension to file because you don't have all of your information together. You file the form with a best guess of what your taxes will look like. The IRS expects changes otherwise, you would have just filed the return.
2. Your brother was alive for 2023 so all income goes on his personal return. However, for filing purposes, he is deceased. You will need to file based on your authority. The IRS has a quick How Do I File a Deceased Person interactive assistant. It is very detailed and may give you additional information.
3. For WI, just attach the Fed 4868 to the WI return. There is no interaction, WI is just casually waiting for 6 months.
If you have an extension for filing your federal return, this automatically gives you a six-month extension in Wisconsin, provided you attach a copy of your federal extension application to your Wisconsin income tax return. DOR Tax Filing Extensions For Paper and Electronically .
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