I’m a financial & medical durable power of attorney for my aunt. How can I protect myself and my aunt from my siblings?

Roberta, I don't know what you mean by they want the checkbook. Is your Aunt still writing checks? As DPOA and if your Aunt is competent to write checks you cannot deprive HER of her checkbook. However, if you have currently been paying all checks and bills, are on her account as her POA and have that relationship with the bank, and your Aunt is no longer capable of writing checks, then having this checkbook is neither here nor there.
In short, your Aunt, if capable can ask for her checkbook if she wishes to have it and can write checks using it as she chooses to do so . IF she is not competent to do this, then you may need guardianship, and accounts may need to be set up so that you, as her guardian pay all bills, including for supplies and care.
If this is a family fight for custody of this elder, no longer competent, this could turn into a real mess; I would consult and elder law attorney. As your Aunt's POA and the one paying her bills all this time the attorney can be paid out of your Aunt's accounts, as this is in the interest of protecting your aunt from possible predatory family.
I am sorry you are going through this, because a war over a failing elder often ends in 1,000s of dollars in a Lawyer's pocket, and no good answers for the poor elder.
Get Counsel.

How can they coerce you?  Do not be a do0rmat?   Tell them no, they can return her to your care or send you receipts

You don't have to give them anything. You have your aunt's POA and that's the end of it. If they now want to "help out" by doing some of her shopping, then they can. This is how to do it. They can pay for her supplies up front themselves. Then give the receipts to you for what they spent and you can write them a check for what they bought her. This is how I handled the household shopping for clients I actually worked for. Neither of them was capable of going to the bank and taking out cash for what they needed, and I refused to use their credit cards. So I paid up front for whatever they needed then gave the receipts at the end of the week and got paid for whatever was purchased.
If your siblings are so eager to help out all of a sudden, then give them something to do. Tell them you don't need any help with the shopping because you've got that handled well enough already, but you'd really appreciate it if they came and sat with her a few hours a week. Or took on some of the household chores like the laundry. Or brought her to some of her doctor's appointments. See how fast they are not willing to help out when it has nothing to do with the handling of money.

Dissent is already there. Sorry for that. Remember that your aunt chose you, only she can change that, and that can’t happen if she’s not competent to do so. You’ll have to stand up to the pressure, maybe with the help of a lawyer.

No matter how you look at it, there will be dissent.

Aunt chose you for a reason, you are about to find out why. Is aunt considered competent? If so, she can change her POA's any time she wants. It is your job to protect aunt from her own children, keep her information private and assist with anything she needs.

It sounds like you need an elder law attorney, if you want to continue as POA. Ask attorney to send a "cease and desist" letter to aunt's family. If they want the power, if aunt is not competent, they will have to pursue guardianship. You can decide you want nothing to do with this and resign, legal notification to her family, then give it to her family to figure out.

Good luck.