My sister is power of attorney and she went and put her name on my mother's bank accounts. Does that mean she gets all the money when my mom passes

There are two considerations. First, as agent under a Durable Power of Attorney your sister can withdraw funds, even all the funds, on your mother's behalf. Second, IF she is "on the bank account" as a joint account holder, she is entitled to half the money when your mother dies.  If the account is "joint with right of survivorship," she is entitled to all of it.  Such accounts pass outside the Will.  But Massachusetts may allow her to be "on the account" merely for convenience.  If so, she is entitled to none of it.