Mike,
There have been many good suggestions made here, and starting with the bank tomorrow will be a step in the right direction.
As has been said, DO NOT allow the original document to get out of your sight. You might very well have to go to the police, and you need to have the original to show them. The bank might or might not act in your best interest. They will, however, act in THEIR best interest at all times. You are a babe in the woods compared to them, so do not accept all that they say until you seek your own counsel (either the police or an attorney or both). The bank might tell you that they have to have that document in order to do their investigation or whatever, but a copy is sufficient to get them started. Remember, if this becomes a criminal investigation then it's for the police to decide what they need for evidence and whatever the bank needs to do is subordinate to the actions of the police. DO NOT give them the original document.
In preparation for going to the police or meeting with an investigator at the bank, I suggest that you sit down and write out the entire timeline of what occured in your family, going back to when your grandparents were alive. State the facts as you know them and avoid making any statements of opinion.
Get someone you trust to proofread it and help you to get it organized, concise and well-written. This will help you to convey the facts as you know them to the police and/or bank investigator and will also help you to recall all that you can, rather than waiting until you are sitting down at a desk during an interview and stumbling to remember dates and other important facts. An organized timeline will also help the investigator because it gives them a lot more to start with.