How do i challenge power of attorney who is abusing his role?

Sometimes what appears from the outside to be financial abuse of the elderly might be something entirely appropriate.  So my first suggestion to you is proceed with caution.  If you've not addressed your concerns directly with the companion, then I suggest that you do that.  See what reaction you receive, what information you receive.  If you have siblings, you might considering asking them if they share your concerns.  Get a unitied front if possible. There's strength in numbers.   Have you talked to your mother?  If you think this person is abusing your mother, you have a couple of options.  One would be to contact your local Area Agency on Aging and report financial elder abuse.  You might also contact the police.  If your mother's congitive function is such that she can no longer manage her finances without help, you could petition to be appointed guardian of your mother or at least her finances.  You'll have to prove her mental incapacity and you'll be able to attack the companion's perceived abuses of her finances in that proceeding