I am financial and healthcare POA for my aunt. She never married and has no children. Am I accountable to cousins when she passes away?

Get ready. Does aunt have an attorney? You may want to meet with them and auntie just to be prepared in case things get crazy. And they often do.

Please consider a care contract for yourself. There are sample contracts on this site. I don't know the ins and outs of setting up a care contract for someone when you have POA over their financial decisions so if it were me, I'd run the contract by a lawyer familiar with the laws of your state.
You are providing care that would otherwise have to paid for. In effect, you are preserving the value of the estate for your cousins.
If you really want to preserve the money for her future use, don't spend the money you are being paid. It will be your money and if you choose to spend it on her care when her money runs out, that's your choice.
Best of luck.

Here is a good article on personal care agreements.

https://www.agingcare.com/articles/personal-care-agreements-compensate-family-caregivers-181562.htm

Your POAs expire when your aunt does. At that future time, if there are questions raised about how her finances have been handled, you may be publicly accountable not only to your cousins but to whomever might have the authority to investigate.

But you will be in no way accountable for any instructions about her property that your aunt has put in a will; and it would be highly improper for you to attempt to influence her decisions about that. Has she in fact made a will?

There is nothing wrong with your cousins' asking reasonable questions about how your aunt's money is spent, and it might be quickest to give them reasonable answers. After all, they're her closest relatives too; I wouldn't assume they all have sinister motives. But this might also be a good time to spell out to them that she has appointed you to act for her and they must back off. What happened about the bank accounts? Did you ask the culprit to explain him/herself?

You already have trouble by the cousins asking for things.

Please be sure everything you do is correct (not that I doubt it is) and is properly recorded. I think a visit to an attorney. at aunt's expense, would be wise. At that time you can look into a caregiver contract if you and aunt wish it. Some relatives get very greedy when they smell money and will go to great lengths and nasty behaviour to attempt to get it. They have no right to anything other than that which is according to aunt's decisions, Be sure her will is in order. Are you executor as well? Good luck!

Mareva, only the financial Power of Attorney can have their name on your Aunt's checking account, which is you. It gives you power to set up a checking account with both your names, so that you can write checks for your Aunt to pay bills and to give your Aunt some spending money.

How is your Aunt doing? What are her health issues? Does she have memory problems?

As CountyMouse had mentioned in her post, when your Aunt passes on, your Power of Attorney is no longer in force. The person who takes over will be the Executor of your Aunt's Will. Hopefully your Aunt has a Will.

As for the Aunt's land, sounds like the cousins want to auction the land, is that correct? What if the highest bid is $5k for land that has a market value of $15k? Are the cousins 100% sure that the land will go to them? Some elders like to give assets to charity.

I don't feel you can tell them how you spend her money. She made u her POA for a reason. If she is cognitive you really don't give them info. I paid everything out of Moms acct. So her statement shows the ins and outs. Hopefully she has a will with an Executor. They definitely don't need to know that info.

Keep good records. If you buy something for her and reimburse yourself make as sure u keep receipts.

I might get a consult with an Estate attorney now to find out what the law requires, as well as some practical advice. Is Aunt still competent? Does she have a Will or has she already set up her accounts with a designated beneficiary or survivor rights at her death? Sometimes, people avoid probate by doing that with their assets. I'd see if that is the case and if she is still competent to execute a Will, because in most places, the POA authority ceases immediately at death. Has she appointed an Executor in writing?