My mother has been diagnosed with dementia. With no power of attorney in place, so what are my options?

Legal Question. This is more of a legal question that i suggest you put to an attorney. However, in the milder stages of dementia, people are generally able to engage in decision making. So there may still be time for her to sign over poa to you.
You can apply. To the court to have her declared incompetent to make her own decisions if this is the case. If her dementia is in the early stages, you can talk with her about creating a power of attorney.