CA is one of the toughest states on income tax, and they will send letters to taxpayers if there is any remote chance they think that person owes taxes to the state of CA.
Even though the mortgage interest statement may have been in your brother's name, all that indicates is that he was the owner or partial owner of a property. The state of CA could only actually tax your brother if he was living in the state of CA or is he was renting out that home and had rental income from the state of CA.
Your brother needs to respond to the state of CA explaining the situation. He should provide documentation of the state where he actually resides. This could include a copy of his drivers license, voters registration card and a copy of his W-2 that shows state income tax paid to another state. He should also include a statement of explanation stating that he is part owner of a home in CA, but that this is a personal property occupied by his sister and is not an income property.
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