@44619herman - might be best to read the preamble to this MI form
sounds like you sister is 'domiciled' in MI and is required to file a MI tax return because she can't satisfy the 3rd condition. Can your brother-in-law satisfy all three conditions? maybe he is still domiciled in MI as well.
Your domicile is where you have your permanent home. It is the place you plan to return to whenever you go away. You may have several residences, but you can have only one domicile at a time (MCL 206.18). That domicile continues until a new permanent home is established. Once established, your domicile can only change when all of the following criteria are met:
1. You have a specific intent to abandon your old domicile,
2. You have a specific intent to acquire a new domicile, and
3. You are physically living in your new domicile.
https://www.michigan.gov/taxes/-/media/Project/Websites/taxes/Forms/All-Years/3799.pdf?rev=d5f7a64f8...).
