As a very general rule, unless the assets of the testamentary trust were included in the gross estate of your mother on her death for federal estate tax purposes (eg, because she has a general power of appointment), the federal income tax basis in the property will not be adjusted upon your mother's death. You really have to speak to the CPA and/or the lawyer assisting with the administration of the trust, or take a copy of the terms of the trust to an experienced CPA or attorney (particularly in fiduciary tax), to get a definitive answer.
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