Legal Question in Wills and Trusts in Illinois

It depends on whether his name is on the account for "convenience", or whether your aunt intended it as a gift.

If the issue is litigated, it will depend on the facts, including but not limited to the signature card on file for the account.

Yes, it all depends on how the account is set up. Also, it depends on what you aunt intends. If she wants your brother to have what is in the account on the date of her death, that should be the controlling factor.