Does power of attorney have to give accounting of monies spent to other siblings?

No. They are under no obligation to provide an accounting. However, if you suspect they are not being responsible please note the following:

An attorney-in-fact under a Power of Attorney is bound by statutory provisions that govern fiduciaries. If they have control over an elderly parent's finances they should be prepared to provide an accounting because they should have nothing to hide. The elder's bank statements should be made available as well as their checkbook so that siblings can monitor the money going in and the money going out and that none of it is being spent for personal use by the AIF. An AIF can be held personally liable for missing funds.

There is a common problem with "family" AIFs who do not take their position seriously nor do they perform in a business-like manner. A POA grants sweeping powers over all a principal's assets. Mishandling of funds can cause a loss of entitlements if the elder has more coming in than they are allowed and someone else is spending it. In many cases the AIF is doing their own shopping while they shop for the principal, combining personal purchases with purchases for the principal, paying their own bills from the principal's checking account or buying gifts "from" grandma for their own children. They should expect to get audited and a savvy family member can and should petition a court to order an accounting.

The AIF should keep copies of checks, bank statements, deposits, check registers, paid bills, receipts and note down petty cash amounts spent on weekly purchases. Anything an adult child does as their parent's AIF should be open for inspection by siblings who are looking out for their parent's best interest. With-holding information, being secretive and being resistant to questioning is reasonable cause for alarm.