Getting financial details. How can we get details about all of her accounts and make sure they are being managed appropriately?

Why aren't you taking care of your own mom instead questioning this caregiver.
You want her to do it for free?

Given the lack of "documentation" that she does not have, and not willing to provide information, give me the chills. I would immediately consult with an eldercare attorney (possible free one hour consultation) and ask how this should be handled. This could be a slippery situation in which you may most likely need some outside help. You could also in confidence talk to the bank and ask them what they think about the situation and how to get help. Good luck. I don't think you can do this one alone.

I believe if none of you are moms POA, your hands are tied at the moment. The fact that your mom now has dementia, means she won't be able to request someone to be her POA either, so you may have to look into getting guardianship of your mom, as that will allow you to take over everything for her. It can be costly to do so, but if you believe moms accounts are being abused in any way, it might be worth it. Best wishes.

How do you know for a fact that she didn't at some point become your mom's PoA? But if she isn't PoA, then funkygrandma59 is correct about guardianship by either the family or the county. You must pursue it through the courts and it can be costly and time-consuming and if the step-daughter contests it, the judge may decide to minimize the chaos for your mom's benefit and assign a different guardian. You can try to contact her county's social services to see if temporary emergency guardianship is a possibility, otherwise you should contact an elder law attorney who does guardianship. If the county gets guardianship then there is no transparency at all into her affairs and the guardian makes all decisions moving forward, including what facility she lives in.

If she is not POA then she isn't legally a fiduciary with the burden of keeping meticulous records. If she was she would not be beholden to you to discuss anything whatsoever, and in fact SHOULD NOT do so as this is private information of your Mother.
If you suspect Fraud it is time to take your evidence to APS and ask they open a case. If you have no evidence you should consider getting guardianship of your demented Mom if you wish to have it. If you do not wish to have it (YOU would then be the Fiduciary with all the burdens of that task in meticulous record keeping. You could also consider becoming the representative payee for your Mom's social security. Look on their site to see what all is involved; it's a lot.
You Mom apparently put her DIL on her accounts, or her name is there since the passing of her husband.
I think ultimately, if she is not open with you and you are worried, you are stuck with seeking guardianship.

Is your mom being cared for appropriately? Does she go without necessities because of her stepdaughter?

If yes and no, and you are not POA for your mom, then it is none of your business what is happening with the money.

Your step sister owns the account, there is no such thing as signatory rights with any bank that I have ever done business with. You are a joint account holder and you have as much right to the money in the account as the other joint account holder.

Obviously, her dad trusted her to take care of his wife after his death or he wouldn't have put her in control of the finances.

What is it that makes you question the situation now?

Consult an elder care attorney, but it doesn't sound like you can do anything. Someone gave the stepdaughter authority to access those finances and the bank recognized it as legal.
There are a lot of harsh comments here that seem unnecessary about what your prior relationship may or may not have been with your mom. None of us has any information that qualifies us to judge that. I hope going forward you can share some quality moments with your mom. In the end those memories are what will mean the most.

If could be possible that she was listed as joint owner on the account. If that’s the case, she can do whatever she wants even close out the account & open a new one.

Before anyone comments this happened to me. My mom had my sibling & myself on her accounts with her as joint owners. My sibling went in & closed the account & opened a new one without my name on it.

I contacted the bank & they told me it was perfectly legal even though it was my mom’s SS & pension checks that were going in there. They told me that any one of us could have done that.

Heartaches, I'm troubled by one of your comments.   This is not a criticism, but it is a commentary and caution on  inaccurate advice.

You wrote:

"Sense (sic) that person is an official signer on the account she can spend the money how ever she sees fit unless it’s proven she knew your mother was not of sound mind . "

This is not true.   DPOAs and POAs, if properly prepared, specify the range of assets covered, the actions that can be taken, and from what I've seen, in enough detail to provide limitations with a given range.  

The fiduciary is not given free range outside of the needs of the primary individual.    No one can spend however he or she sees fit, regardless of mental status.   And certainly no one can lawfully spend funds on herself.

Imho, perhaps you should retain an elder law attorney.