Insurance companies are quite good at determining if someone has the legal right to change beneficiaries on a life policy.
When the original owner of a policy passes away, there are generally provisions written into the policy which allow for secondary owners or for the executor/executrix of the original owner's estate to make further changes (such as beneficiary changes).
1st edit: Stuart and Max's Mom are incorrect. In a life insurance policy, policy changes are always made by the OWNER of a policy, which can be a different person from the insured and also be a different person from the beneficiary. So long as someone has an insurable interest, they can own a life insurance policy on another person (but only with that person's signature on the application).
In addition, in a life insurance policy, the insured, owner, and beneficiary can be and often are three separate individuals.
2nd edit: As executrix of your grandmother's estate, your aunt had the legal right to assume ownership of the life insurance policy and to change the beneficiary. While you may have disagreed with your aunt's decisions and communications regarding your mother's life insurance policy, her actions were well within her rights as executrix of your grandmother's estate.