If you are certain of your facts - i.e. that there is a will and that you are an executor, & that she has misled the bank, then you need to take action. First, go to the bank and show them the will. Insist that they do not take any action without your consent. Ask for this to be confirmed in writing. Get the name of the person you see about it & keep a note of it. Ask for a written acknowledgement at the time that they have received a copy of the will.
Second, let your sister-in-law know you have done this and that you will similarly consest any other actions she may have taken which are not in accordance with the will.
Tell her that she either co-operates with you, or you get legal advice about having her removed as an executor on the grounds that she has lied to the bank and is trying to exclude you from acting.
All the above assumes the estate has sufficient value to make it worthwhile - legal advice and action in a case like this could be quite costly.