Call a lawyer in that state and hire them to handle your case for you. If they're trying to assume control over your mother's finances then something is amiss. You can also call the courthouse in the county where they reside to confirm if there is in fact a conservatorship for your mother.
This must be a very stressful situation for you. Ms. Johns provides you with excellent advice. It does sound as though you need to act to protect your mother in light of her dementia and your uncle/his family’s actions. As he and other family are placing their names on your mother’s accounts when she lacks the capacity to authorize that, she is a victim of financial elder abuse, as well.
You are correct that a conservatorship would be the best means to protect her. If you are appointed as your mother’s conservator of her person and finances, then you could determine where she lives, as well as manage her finances. However, as Ms. Johns points out, the first order of business is for you to find her. You cannot obtain conservatorship without the court having jurisdiction over your mother. This means that the conservatorship (called a guardianship in some other states) must be brought in the state and county where your mother is residing. I agree that you will need to hire a private investigator. I would also add that you could also contact adult protective services in the county where your uncle/family’s last known address is. It is possible that someone else has filed a complaint against your uncle/his family on your mother’s behalf. If not, you might consider filing your own report with them.
You’ve also stated that your relatives may have moved to Florida. Florida is one state that uses the term “guardianship”. I am attaching a link to their Guardianship guide. A Florida elder law attorney can assist you regarding whether you would need to be a resident in order to qualify as your mother’s guardian. It might also be possible to have a temporary conservator/guardian (such as a private fiduciary or the public guardian) appointed for your mother in the state where she is living, with the goal of having the guardianship/ conservatorship eventually transferred to California, with you as her conservator. Again, first your mother needs to be located and then you’ll need to proceed with your action. Depending on the state, it’s possible that the public guardian and/or adult protective services may need to be involved from the onset.
I hope this information is helpful for you.
Disclaimer: The above answer does not create an attorney/client relationship. My responses are intended to provide general information about the question posted. I am licensed to practice in the state of California. The information provided on this site should not be used as a substitute for conferring with or hiring a competent legal advice from a licensed attorney that practices in the subject area in your state.