Can you undo DPOA? How?

Phooey. It sounds as if once someone is assigned DPOA, then there must always be someone who has that. This is in FL, so not sure if that matters. Mom still signs things, I just have DPOA for when she's not available right then or if she becomes incapacitated.

I cannot get over the way my mother, sister, and brother in law are now behaving. It completely broadsided me.

No, just tearing up the document will not work. Your mother will need to revoke making you her POA and make someone else her POA.

If the POA was drawn up by a lawyer you'll need a revocation and another person to sign as POA.