My cousin's sister put their mothers dividend check for the estate into her own account. What happens now?

It was a mistake of the bank to allow deposit of the check into the account of someone who was not the named payee. I would start out by going to the bank, first. If you cannot get any resolution from them (i.e., reimbursement of the full amount of the check), then you must demand reimbursement from the sister. Unfortunately, if she refuses, you will have to hire an attorney at least to send a demand letter to the sister. Frequently that is sufficient to get the desired result without having to go through the costs of a full-blown legal action in court.

How did the sister get hold of the check?

And to whom was the check made payable? Did the sister forge her mother's signature? Some banks, however, don't require endorsement of checks unless the depositor is getting cash back.

Are there any accounts which the sister and mother hold jointly?

The Executor has to issue individual checks to each heir. Now you have to explain what you did to the probate court.

Are you saying daughter has POA for her mother so she was able to deposit her cheque? If that is how it happened then it seems to me you did everything you were supposed to, the problem is beyond your scope of responsibility. Unless you want to push the issue?

There is no POA. The mother wants her money.

I would say contact APS immediately and even the bank where your sister put the money. If you have to show up on their doorstep, do so. I would also make a police report as well

What kind of dividend check? Maybe the daughter has a bank account with her mother. Have you tried talking with her about this matter? Maybe she intends to give her mother half the money. Communication, communication, communication.

Wait a minute. I have specifically asked this question of Chase. The computers run checks deposited and do not care what or who the addressee is, only that the money from the check giver clears. It doesn't care what the printing says, only the numbers at the bottom of the check.

You're going to have to hire an attorney or should hire an attorney.