Is this a small estate, or subject to probate?

If the latter, her brother is an heir but not necessarily a legatee. Heirs have rights, including the right of notice. Has he received Notice of Probate? Did she die intestate? If not, has he seen the will? He has that right, too.

Evil Aunt has access to the accounts for one of two reasons. Either she was a joint account holder or named beneficiary (1), or she was authorized by a court (2).

In the former situation, the account becomes her account, and is not part of the estate.