The Will governs only what your mother owned in her name alone on her death. If your sister's name was on the account, then she owns the balance. The only way she could have been on the account with authority to write checks would have been to be a co-owner. Jointly owned accounts are not governed by a Will or go through probate. They are automatically owned by the second party on the death of the first. The provisions in the Will apply only to the extent your mother individually owned the funds available to make those bequests. If there is not enough after probate administration, then it is divided on a proportionate basis.
This comment does not create an attorney-client relationship. The law and its application by the courts is constantly evolving and changing. This discussion is not to be taken as a definitive guide, and should not be relied upon to determine all fact situations. Each set of facts must be examined separately with the current case and statutory law analyzed and applied accordingly.