My sister is financially abusing my mother. How and at what point do I become trustee?

It sounds like sis may have already done a number on the assets. I'd talk to my CPA who has a forensic accountant if I could not dig up the info myself. You may need to sue sister for what's been removed if it's not prosecutable. The other issue is the money for conservatorship/guardianship is often paid for out of the patient's funds, not the conservators. It's for the benefit of the elderly, not for you, that's for sure. I'd price some other attys, but don't use the lowest one. Go by feel and CPA's recommendation - the CPA will be working with the atty to file papers for the courts, so they know who is easy to work with. 

She lives at home. The assets are not being used for her. Annuities, cds, savings

If the assets are NOT being used for her care, I would report it as a crime. BUT if sister has a written caregiver contract drawn up, well that is a different story. If the written contract calls for her to be paid, it is legal as long as the wages are reported and taxes are paid.

What type of assets are being removed from the Trust?  I see from your profile that your Mom lives at home, who is her full-time caregiver? 

Is there enough money in the Trust for Mom to have around the clock caregivers once your Mom's dementia gets worse?   If your Mom needs to go into Medicaid to help pay for her care, then Medicaid will do a financial 5 year look back and wonder where did those assets go.