I am sorry for your loss.
You will need to contact the broker who issued the 1099-B and let them know of your mother's passing. They may issue you a corrected 1099-B with the allocation between before her passing and after her passing.
If they do not issue a corrected 1099-B, then you will need to determine what portion of the investment income being reported on the 1099-B (or 1099-INT/1099-DIV) relates to her final income tax return and what portion relates to her estate return and report this income on the correct tax returns (Her personal on Form 1040 and the estate on Form 1041).
