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).
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).
In addition you will need to issue 1099s from your mother to the trust for it to use on its 1041. (assuming it had more than $600 in gross income in 2017). The trust became irrevocable after your mother passed and you need to get a tax-id # (EIN) for it and may need to file a 1041 depending upon its income and if the trust continues or if it ends.
You should probably review IRS Pub 559 in some detail and consider consulting with an estate/trust attorney, or a CPA or enrolled agent who specializes in trusts and estates (most don't). <a rel="nofollow" target="_blank" href="https://www.irs.gov/pub/irs-pdf/p559.pdf">https://www.irs.gov/pub/irs-pdf/p559.pdf</a>
[EDIT] See in particular Pub 559, page 5 "Income to Include" and "Forms 1099"