This is a difficult question and one reason to avoid joint accounts. While joint accounts permit the joint owner to manage financial affairs for the original owner, they also remain the joint owner’s property when the original owner passes away. However, in many cases it’s recognized that the purpose of the joint ownership was to facilitate bill paying and financial management and not as an estate planning measure. In those cases, the joint owner usually cooperates and adds the account to the estate to be distributed in accordance with the deceased person’s expressed wishes. Where that doesn’t happen, you have the matter of proof. Unfortunately for you and the other grandchildren, the burden of proof is probably on you to prove your grandfather’s intent was not to benefit the other family member.
You have the added complication that the joint owner is also executor. She has a clear conflict of interest. As executor, she has a fiduciary duty to act in the best interests of the beneficiaries under your grandfather’s will. Acting in your best interest would be to include the joint account in your grandfather’s estate to be distributed to his grandchildren. While it’s not clear whether an independent executor would be successful in that regard, it’s difficult to see how the family member can act as executor if she is insisting the joint account is hers.