My mom and myself are joint signers on an account and my sister wrote a check out of that account. Is that allowed?

Your name is on the account. Your mother's name is on the account. No one else's name is on the account.

Someone wrote a check from that account who is not on the account and signed your mother's name.

Do you think this is legal? If I wrote a check, forged your signature on it, and cleaned out everything in your account, would you ask if it was legal for me to do so?

My guess is you'd call the bank and file a police report. Do that.

In the future, you'd do well to have the checks changed to your mother's name AND yours. Which means checks can't be cashed without your signature as well as hers. Also, I'd keep far less than $4,200 in any account your mother can access from AL.

This is fraud. STEALING. FORGERY. THEFT.
This is elder abuse.
Tell your sister that the funds get returned ASAP--And that you are heading NOW to the bank and to the police.
Who is POA for Mom? You say you and she are on this account? How DARE she!!!
CALL THE BANK NOW TODAY AND REPORT YOUR SISTER JUST FORGED YOUR MOTHER'S name and ask how to move forward. Go in and get copies of the forged check. Fill out their papers with a bank officer.
Go to the police and fill out their papers.
Ask for their help in doing so.
Do this even before you speak to SISTER.

Your SISTER just STOLE MONEY from your mother to the tune of almost 5,000.00. I don't understand that you don't understand she just stole your mother's funds and forged a check.

I wish you luck. Call the bank. Tell them what you told us.
And please, after all THIS, do not come back to tell us that Sister is caring for Mom and is Mom's POA.

Forgery and theft are criminal offenses, call the police and press charges.

Report the theft to the sheriff/police and your bank. Press charges.

Close the account, get a new one, don't send any checks to your mother, she obviously doesn't know what is going on. Dementia?

It's a felony crime. I would report your creep of a sister to the police. She hopefully will spend time in jail in OK. And kick her (probable) drug habit while in the cooler.

When you notify the bank, ask about the laws governing this offense. Ask that they notify law enforcement. Once sister does something like this, she may do something else of the same nature. She needs to be scared enough never to do it again.

Until this is cleared up, as a preventative measure, have you stopped any other bank accounts of your Mom's that your sister has access to?

Have you asked your sister directly about the cheque & money?

She helped herself to your mom’s money. Tell the bank. You could also report her for elder abuse if your mom wants to take it further.

I am so sorry that this happened to your mom.

Forging a check is a serious offense. Report it to your bank immediately, most banks have a 30-day limit for reports like this.
As long as your sister does not have POA and is not on the bank account, she is not authorized to sign checks. Contact your bank and they will provide you the proper paperwork to complete.