You are right. When you and your siblings inherited your mother's estate you became equal co-owners of the copyright in your father's memoirs. Co-ownership of a copyright is very similar to being a tenant in common of a piece of real property with one or more other owners. Since there are four of you, you each own an undivided 25% interest in the copyright in the work. Each co-owner of a copyright has the legal right to enter into nonexclusive licenses with others to publish or otherwise exploit the work without obtaining permission from the other co-owners. However, the co-owner must provide the other co-owners an accounting of any profits earned upon request, and share the profits according to the ownership interests. Some courts have held that co-owners may not enter into exclusive licenses without obtaining permission from the other co-owners--- for example, you couldn't grant a movie studio an exclusive license to create a movie based on the memoirs without the consent of the other owners. But, this is not what you've done with Amazon--the Createspace publishing agreement used by Amazon specifically provides that the agreement is nonexclusive. Since the publishing agreement with Amazon is a nonexclusive license, you had every right to enter into it without permission from your siblings. Their rights are limited to receiving an accounting from you and their 25% share of any royalties you earn. Likewise, any of your siblings may enter into nonexclusive licenses on their own to publish the memoirs; but they may not use the footnotes or epilogue you wrote without your permission--you are sole copyright owner of these elements you added to the original memoirs.
By Dear Rich staffer Steve Fishman, author of the The Copyright Handbook.