What should I do? Cousin on my deceased stepdad's side took over mom's finances and made himself power of attorney. She has dementia.

Do you have proof that this person is "scamming" your Mother? If so the legal system is the way to go. I would visit an elder law attorney to find out what is the best way to proceed. There are ways to have the POA's records checked to see if his care of this relative is correct and appropriate in the eyes of the law.

You can report financial exploitation of a vulnerable senior to APS and local law enforcement, however, you need to have proof that something inappropriate has happened. If not you are only creating a further problem with your mom and her as of now, legal representative. Also, your mom would have to say something about being concerned if she is verbal and can be talked with.

If she defends her POA, then you will most likely get no where.

I would consult with a certified elder law attorney and find out how it can be verified that mom is being taken care of and not being financially exploited.

Austin, QUICK answer is take him to Court!
He will have to present all records regarding his handling of her finances, every single penny.
Get a letter from her doctor as to when she was diagnosed with dementia, what treatments and medications he prescribed for her.
If the letter provides a date prior to the date your Mother signed the POA which should have an attachment she would have been required to sign stating she was doing this of her own free will and understood what she was signing, there's your answer that he is taking advantage, especially if his records do not back up his claim.
In most states, whoever wins the case, the other must pay all attorney's fees, Court fees, and hrs, recording fees, etc.
It will be worth your time and money to hire an attorney too. The attorney can request the records from his attorney should he have one OR request through the Court by serving him with letters of demand which will specify the timeline to get everything to the Court and your attorney.
Make sure your attorney requests a very short time period so as to rattle him to the point he's like a chicken that just had his head chopped off.
I'm fighting my ugly SS right because I am trying to get an asset protection divorce for my Mom. Ugly SS has been fighting me for over a year, the biggest reason is she hasn't paid a single penny toward the Community debt (Community property State). SS wants my Mom to pay for her Father's care! Both are in assisted living due to Alzheimer's and Daddy NEVER saved a penny for anything (long story as he is a sweetheart).
My Mom worked hard all of her life, saved a percentage of her earnings and invested well. All before the marriage so none of it is his. SS does not like the state laws regarding community debt. So far she as his POA and Guardian with limited rights owes my Mom almost $100.000 all because she is pissed because Daddy isn't on the deed to the house (Mom's sole/separate) so he doesn't have any monetary rights EVEN though they have been married almost 27 yrs. SS thinks he should because they have been married for such a lengthy time, and to think there is a University that gave her a degree in elementary education!
SS has been so irate that she has committed fraud 6 ways to Sunday. I've discussed with my attorney to be ready with the paperwork so I can press charges for Bank fraud, ID theft, knowingly providing fraudulent information to the Federal government. My attorney cannot legally threaten with charges, but I can if she does not agree with my settlement 'requests'. SS will have a choice of door #1, #2, or #3. It doesn't really matter which one she picks because they all have jail/prison time, fines or both. My favorite is the Federal because she could get 5-15 yrs in prison, $100.000 fine or both.
BE SURE to research state law and statutes. It's really fun to inform your attorney that they're wrong regarding an explanation of the law. My attorney has told other attorneys involved that I'm the most organized and knowledgable client she has ever had. RESEARCH.
One area no one ever researches are the rules/regulations for notaries. These actually change quite often. THIS COULD BE YOUR FAIL SAFE NET!!
RESEARCH THE COUNTY RECORDER'S WEBSITE TO SEE IF OTHER DOCUMENTS HAVE BEEN RECORDED I.E. THE HOUSE TO HIM UPON HER DEATH.
You're facing a very big battle, just be ready. Turn over every stone. If you know or have a friend who is an attorney, ask for help, direction or an attorney who can get the job done.
Best of luck and buy stock in Kleenex!

You do not need a lawyer my friend! You have a piece of paper stating that your mother has dementia. That was five years ago! I'm sure she has deteriorated more since then. You can file papers in court yourself and prove that she is not capable of understanding what she is signing. You can also schedule a doctor's appointment with an organization that does dementia testing and get a fresh test. You need to gather as much evidence as possible proving that she does not understand things. Find out how to file with the court on your own and bring all of your facts with you.
In court Do not get emotional! Judges do not care about tears they care about facts. I have no doubt you will win this and you will not need to spend $5,000 on an attorney. I have been to court myself so I speak from experience. I could not afford an attorney and have won three cases because I had stacks of proof. You can do this!

Thank you for your advise. I went to lawyer but he wants to charge $5000.00 just to start case. I just don't have it. I will try office of aging.
Hhope you get your case resolved sounds really terrible. Good luck!