Sister & I act jointly as POA for mom. Sister has cancer, keeping it secret (from me). Is she obligated to tell me?

My sister won't speak to me because I have kept her from getting money from my mother. Mom has dementia and is incompetent. I just found out that sister has Stage 4 cancer and is not accepting treatment. Does she have a legal obligation to tell me? Per my mother's POA, I become primary POA if she ceases to act.