My parents are asking me to give their money to my sibling who is taking advantage of them. If I have Power of Attorney, can I refuse to give them money?

Yes, you can!

Sounds like a Family Meeting is in order here. The top priority for your parent's money right now is that it be used to take care of them--especially if theymay need Assisted Living or Long Term Care that is not covered by Medicare or their LTC Insurance. Even if your parents were to "give their money" to a sibling, most States have a reach back period of 5 to 7 years to claim that money back for their care should they need Assisted Living or a Nursing Home and not have the money to pay for it. This means that if your sibling got x-amount of dollars and one or both of your parents needed public assistance for Long Term Care, the State could demand the gift money back from your sibling. This means the full amount--not what they haven't spent yet. It also means that a lien can be put against them until that money is returned. This varies from State to State but you should consult an Elder Law attorney before or in conjunction with the Family Meeting so that everyone is clear on their responsibilities. This includes your parents who may not be aware of the consequences of giving their money away without planning for their continued care. And in answer to your original question: Durable POA only goes into effect when your parents are unable to manage their own financial affairs, for example your mom has dementia and your dad who is her primary caregiver, suffers a stroke. In that case the person designated POA would be authorized to act on their behalf to pay bills and make other life decisions. You cannot prevent your mom and dad from giving their money to your sibling as long as both of them are of sound mind and able to make their own decisions--even if they have given you Durable POA. That is why you need to get the entire family on board the Reality train.

As long as their bills are paid and they are in want for nothing, it is THEIR money, you should do with it what they want making sure their needs are met first.

I agree with EZ...do your best to keep grubby little fingers away from your parent's assets. Very often a parent gives their POA to a responsible child so that they do not have to say "no" when an unscrupulous relative asks for money. So go ahead and be the bad guy and say no to the sib. You are protecting their assets that cannot be replenished if the sib renigs on the "loan." However, a POA will not prevent your parents from giving away their money.
I am in the same situation. I could not convince my Mom that throwing "good money after bad" was not a good idea. I recommend having their assets be placed into a living trust with you as a trustee. The trust becomes its own "entity" and you have say over how funds are dispersed. In essence your parents are saying to the world that they entrust you to make these financial decisions for them. It will give you peace of mind that all her hard-earned assets will remain theirs.
At this age, parents often do not understand how much it takes to provide the services they will need so they agree so easily to pleas of financial help from family.
Before sitting down with the fam, I would contact an elder attorney and find out the best way to protect your parents.
good luck...just another thing caregivers have to deal with!
Lili

Contact APS adult protective services. Fiduciary Abuse is a form of Elder Abuse.
As a POA you are basically empowered if something should happen to them and they were unable to be make their wishes known (coma, hospitalized, etc.). So if they are making their desires known then you cannot interfere - but you can say "NO, I won't do that, if that's what you want, then you go ahead".