Assuming your mother had the power under the trust to make the change you describe, your stepsister is no longer a beneficiary of the trust and has no right to that information. However, that’s an answer related to her rights under the trust. She may have a claim if she can establish that her father and your mother had an agreement that your mother violated in amending the trust.
Depending on the circumstances, your stepsister may have a claim for a breach of this agreement and could in that case ultimately obtain the information she is requesting. You may or may not want to work things out with her.
Harry S. Margolis practices elder law, estate, and special needs planning in Boston and Wellesley, Massachusetts. He is the founder of ElderLawAnswers.com and answers consumer questions about estate planning issues here and at AskHarry.info.