Is it possible to change the primary on a living will to the secondary person?

My brother is primary because mom never changed the living will. I am secondary. She's being tube fed because "he" wants her to be. He hasn't seen her in 7 years and she lived with me. I know I should have had it changed but I thought because I was on it, everything would be okay. The hosp and nursing home won't even listen to my views of taking out the tube. Mom is virtually a vegetable now and knows nearly nothing. Cannot walk or sit up or eat or drink. Brother will not pull the tube and it has nothing to do with religion as he's atheist. I have NO idea why he's doing this to her as he won't say.