Is it possible to place a lien on someone's bank account if they owe you money? If so, how?

You are going to have to file some papers in court. It depends on where you live, but you can find the information online or go to your local courthouse and speak with the clerks. They will be able to assist you in which forms you need to fill out.

I am assuming that a Judge has ordered them to pay you a certain amount and this has gone through the court system. But, if your sister owes you $4,000 and you haven't taken her to court, then you can't place a lien or garnishment on her bank account.

Right. It all goes through the court. You have to 1st get a judgment against the person then file for a lien against an account if you know the account info.

If it's an amount within the limit of small claims court, then both steps are easy to do yourself.

How do I place a lien against property(land and house) inoregon?
What form do I use. These people owe me a shed orthe money back. It is 4500.$