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

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.

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.