DBA infers that she is doing business in the U.S. A person with a B1/B2 visa cannot do anything that generates income or even "volunteer" in a position that would otherwise be a paid position while in the U.S.
I think it is a real stretch to say that she owns the company in Mexico but will not be carrying out any prohibited activities (i.e. work) in the U.S.
Her borther should be the only one listed on this account since he is the only one allowed to generate income for this business in the U.S. I would not allow her any direct access to the account: knowing that she is "working illegally" and helping her in the situation might cause you some problems.
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Opinions are my own.
"I was gratified to be able to answer promptly. I said I don't know." - Mark Twain