Hello: FYI I am Certified as a specialist in estate planning trust and probate by the State Bar of California.
Let me take this in steps:
(1) Just because your mother had a trust does not mean she did not have a will. Normally we prepare what is known as a pourover will and a trust together - The trust being the beneficiary under the will for any assets not titled in the name of the trust at your mother's death.
(2) Under Ca Probate Code section 16061.7 you are entitled to a legal notice from the trustee if you are either a beneficiary or a person who would have been an intestate
heir if your mother died without a will or trust, and are entitled to request a copy of the trust. You, as a daughter, qualify as an intestate heir even if you were not named as a beneficiary under the trust. It would be a little unusual if you were not named as a beneficiary. Notice is supposed to be given within 60 days of your mother's death.
(3) If you do not get the legal notice and a copy of the trust / will upon request, I would hire an attorney, to make sure that the trust and estate is properly handled.
(4) If you are a beneficiary you would be entitled to an accounting by the trustee but you should check the language of the trust with respect to accounting by the trustee.
(5) Lastly if the assets titled in your mother's sole name (not titled in the trust name or not subject to a beneficiary designation) at her death exceed $150,000 then a probate will be required for those assets whether or not there was a will.
(6) If your sister does not give you the required legal notice under Probate Code section 16061.7 and a copy of the trust on request then hire an attorney without delay and don't wait around for several months as the handwriting is on the wall so to speak.