How can I get information about sibling's apparent takeover of finances and a "new" will of a parent?

IF you have proof that sibling is getting large sums, call the cops. Elder financial abuse is a crime. It is also possible mom is delusional, that none of this ever happened. If so, you immediately petition the court to appoint someone as a Guardian for mom. Better safe than sorry.

Since you have the name of the new attorney, contact him or her and ask for a copy of the Will. Be prepared to submit identification as attorneys generally are not going to release confidential information based on a phone call. You could even ask for a meeting to have the Will terms "explained" to you - that would give you an opportunity to see what the situation is.

It's not clear to me where your mother is living. If with you, or you with her, then your sibling must be making some secretive contact somehow. See if you can find out what's going on in that aspect.

Your sibling apparently is a joint signatory on the checking account if she's writing checks, which I assume since she "has the checkbook."

I'm wondering if your mother is a bit confused about the situation and is just listening to the sibling's "advice"?

It does not sound reasonable for you to be doing the caregiving in your or your mother's home and not evidently have Durable or Medical POA. Is your sister going to pay your mother's medical bills from your mom's money or does she expect you to do that?

Also, with you doing the actual hands on caregiving, you need medical POA for your mother so that you can discuss your mother's medical care with her doctor. If you don't have that, then you will be hindered in that area.

For your sister to have taken over your mom's finances she must have been given Durable POA which may be on file at the clerk of court's office in the county where your mom lives. You may be able to find this online if it is on file there.

Your mother did not need to make a new will because the lawyer who wrote it died. That raises a red flag.