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Technically at this point in time neither of you have a "right" to know anything about her assets. An executor is an individual named in a will to "execute" the provisions of the will. The will does not become effective until the testator (the person making the will, i.e. your mother) dies. At that point the will is "set" and the executor can then file the will for probate
. The executor has an obligation to provide an "accounting" to the probate court, which represents the deceased individual's assets at the time of death (which become the "estate" that is distributed to the beneficiaries of the estate).
Now if he doesn't file for probate, you can still do so. Once you file, he can either step up and choose to be named executor, or waive his rights to do so and you can be appointed executor by the courts. Either way, once probate is filed an accounting will need to be done, at which point you can learn about the assets that your mother had at the time of her death. Until then, however, neither you nor your brother (at least in his role as "executor") have any right to know this information, as she is not yet deceased.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for my answer unless and until you rate it a 3, 4, 5 (good or better). Thank you, ***** ***** luck to you!