My grandmother has appointed my oldest aunt POA but my grandmother still can make decisions on her own. Can my other aunt's contest her POA?

My grandmother has appointed my oldest aunt POA but my grandmother still can make decisions on her own can my other aunt's contest her POA she doesn't care for my grandmother my youngest aunt died has always cared for my grandmother the oldest aunt only wants to step in where her (my grandmother) money is concerned my oldest aunt has a gambling addiction has taken my grandmother's money placed my grandmother in a nursing home without talking to her other siblings she can't be in the same vacinity as my grandmother for more than a an hour she don't care. My grandmother just found out that she has cancer and my oldest aunt is already saying that she's gonna take my grandmother's ashes to New Mexico to bury her with her mother and not let her siblings have a proper funeral service what can my aunt's do to stop this from happening our grandmother is our world. Please help us.