Siblings are taking things bequeathed to others. Is this legal?

They are crimes and should be reported to the police. Moving large amounts of money to your own account is a direct conflict of interest, it is not inheritance, and it constitutes good grounds for removal of a Guardian.

Here is what I'm envisioning. SisX realizes that CousinY is going to get the valuable painting in the dining room upon Mom's death. She visits Mom and goes on and on about how much that wonderful painting in the dining room has always meant to her. She'd love it in her family room. Mom (who perhaps can't remember the painting at all or to whom she intended it to go) says "go ahead an take it, dear."

Is that about the size of it?

Depleting Mom's assets while she is still alive may make it harder for her to meet her own care needs and if she needs to apply for Medicaid this might be considered "gifting" and result in penalties. So it isn't just about cheating CousinY out of a future inheritance, it is harming Mom now.

As her guardian you have a responsibility to protect her financial interests. If a serious heart-to-heart with your siblings cannot resolve this in Mom's best interests, you need to take legal action.

They are both unethical and a crime..

They are stealing from your mother.

Someone seriously breached your mother's trust by providing copies of the Will to the siblings. That person obviously hasn't been loyal to your mother's wishes.

Immediately secure the house; change the locks and take inventory.

Close what remains in the bank account and reopen it, perhaps at a different bank. Make it clear to the bank management why you've had to do this, as you'll expect their cooperation in the hopefully ensuing investigation.

And don't tell the other siblings where you've transferred the funds.

Do contact the police; unauthorized removal of funds is theft, perhaps grand theft, depending on the dollar amount. If this was the sister who's co-guardian, I would think it's ground for removal.

I still don't understand why anyone would breach your mother's trust by relesing copies of the Will. And if you knew about these activities beforehand, or were the person who gave them copies of the Will, you may have breached your fiduciary duties.

If they didn't take anything that wasn't bequeathed to them, I'm not sure how the issue of conversion arises. But the fact that they didn't makes it somewhat simpler, legally, but not emotionally.

I think they've given you a glimpse of what their attitudes are, that they couldn't wait for your mother's death to begin taking what they would inherit.

It's a sad situation, no question, and I can understand your hurt. I experienced something similar when my sister died. It was a real shock and wake-up call. I try to think in retrospect though that I now know what will happen when my father dies, so I have advance notice on how to prepare for and deal with it.

Try thinking of yourself not as a daughter but as someone who's handling your mother's affairs as a professional; make the connection dispassionate, view your siblings as heirs and not siblings. Distance yourself from them emotionally, don't think about them or worry about them. They're going to pursue their own interests regardless. Concentrate on providing and getting quality time with your mother.

Sorry it has to be this way; sometimes families can really be a disappointment.

Since your mom has dementia what your siblings are doing might be considered financial abuse. Someone here with more experience than I will probably weigh in.

It's definitely unethical, immoral, and exploitative.

How low can they go? I would lock up everything and demand that they return what is missing.

A bequeathed item still belongs to the mother UNTIL HER DEATH. Taking it "early" is stealing. If mother would happen to need some of those assets before she dies they will do her no good if they are already gone.

And these sibs are not taking their own inheritance -- they are taking things Mom had designated for others.

By the way Who the heck sent them a copy of her will?

I have never heard of a co-durable power of attorney nor co-guardianship. You actually went to court to get that co-guardianship? That's the only way you could have it. Please read your power of attorney over again. Normally, it will name an agent and a successor agent. That's not the same as "co-POA". The named agent is in charge until she can't or won't be anymore.