My Dad and Mom had a written will completed by one Lawyer-Law firm in which it basically dictated in the passing of either my Dad or Mom, the other would inherit everything & that any distribution to the children would be done as fairly as possibl...
It is common for a spouse, your mom in this case, to review their estate plan when their spouse has passed away, your dad in this case, with the same lawyer or law firm. From what you stated, your mother appears to have inherited all the marital property from your father, which is also one of the most common distribution schemes.
When your father passed, your mother's estate plan has become somewhat out-dated and she may have wanted to or was advised to update it, also very common and the prudent thing to do. As a child or even a beneficiary, you are not entitled to a copy of your mother's new estate plan. However, I do advise my clients to share this information with their family since it helps them to plan for their own estate and it also helps to avoid surprises and family feuds after a family member passes.
In my experience, it is not uncommon for only one child to be named as the primary Attorney-in-Fact under the Power of Attorney and the Executor (or Co-Executor) and other children to be listed as successors. I would not advise a parent name that many (in this case all 5 children) Co-Attorneys-in-Fact or Co-Executors. So, I would not assume undue influence from that fact alone.
However, if you still believe your mother is subject to undue influence I would suggest trying to sit down with your siblings and mother to see if you can have an open and honest conversation. If that is not an option, I would try talking to a mediator (a neutral 3rd party) who could help facilitate a conversation about this delicate subject before attempting anything as serious as guardianship or a conservatorship which could have many negative outcomes for your mother, the estate, and the family as a whole. There are pro bono community mediation programs, if cost is an issue, that could help your family through this difficult situation before the conflict escalates any further. Best of luck navigating this sensitive matter.