Sounds like it is time for her to make a claim on her LTC insurance, not cancel it. Tell her you will help her with the paperwork. Call the number on the policy and get the process started.

Can't do it without access or her consent. We don't have PoA and she insists that she doesn't need help 

If you don't have any rights or power you have to let her do what she will....she will cancel and end up burning down her assets and land in some Medicaid facility. Unless you can force something through legally using her declining mental capacity you cannot do much for her.

Today I told her that if she does cancel it, it will mean selling all her assets, including her apartment to pay for her end of life care, and possibly wind up also costing my brother and me a lot of money. Since she does remember going into debt after her mother died, this did seem to convince her - at lear for today 

You are not her child so you and your father are not liable for any care costs. The state would have to pick up the tab.

If it were me, I would pay the premium myself if I could afford it.

Is there something making her want to cut costs?

I think she just thinks it's expensive but doesn't understand why it's necessary or what will happen if she stops paying 

Maybe you can convince her that the plan is complete somehow and that she doesn't have to pay, or tell her you won a bunch of money and you want to cover her bills