What are my legal rights in behalf of my mother (diagnosed with beginning stages of dementia) if my siblings are taking her money?

Are your siblings helping with caregiving? Perhaps if this is the case, your mother is giving them money for caregiving.

Your rights? None. Your mother can give it all away until you pursue Guardian status in a court of law. If you think she is incompetent, see an attorney and petition the court for a conservator.

Time to talk to an Elder Law attorney so that you can somehow get control of the situation.

Who is your Mother's financial power of attorney? Since your Mother is in the beginning stages of dementia she probably can still appoint you or someone else to be her financial power of attorney. A financial power of attorney can gather up all the financial information and have control over what bills are to be paid, write out the checks and pay said bills.... and to make sure no one is taking money from your Mother.