Does a more recent Power of Attorney void an older one ?

My sister-in-law was appointed power-of-attorney for my mother- in-law. Since then , she has refused to assist in her care in any way and has only seen her twice for a couple of hours in the last two years. my mother-in-law is deaf , blind , and can no longer walk at all and has final stages of dementia. In addition she has lost control of bodily functions , having to be fed and bathed and speaks very seldom. She lives with my wife and I full time.