How can you keep a bad sibling from continually praying on your parents financially?

You can also get a vulnerable adult protection order. My sister is a drug addict and I may have to do the same if she tries to contact my father for drug money again.

Your mother does not sound that she is competent to conduct her business in a business like manner. Thus, without a POA, the only other alternative is guardianship to protect her.

Thank you everyone for your helpful comments. My mom thinks she is 20 so I doubt she would give me POA. I am afraid of the thief who lives next door, rent free, thanks to my real estate brother,

Get on your parents account as a joint account and get the financial POA. That is what I did. I kept mom pretty involved in the financial decisions until she no longer could make them and I was free to do it myself. Worked for me. I had a disaster on my hands too, but I was able to get it resolved and stopped my low life alcoholic brother from taking her money and using my deceased father's credit card (they have the same name).

I cant say that this is recommended but is quite funny. A friend of mine was having the same problem and his parents had money put away but told the moocher that their money was becoming really tight. One day the moocher came and wanted some money and guess what...dad wrote a check. My friend always kept track of the checkbook so he knew how much was in there....well, he let the check bounce!!!!! Well, buy the time the sibling had to pay for his own bounced checks and was told that the money just isnt there any more....the moocher hasnt asked for anything again...so far.

Ask your parents if they have a will, if so ask to see it (often durable POA is assigned when they draw up a will, I didn't know I was POA until I saw the documents attached to the will), if no POA is assigned convince them their will needs updated and have POA added. I also was added to my Mom's bank account, and from reading these threads it may very by where you live, but I took the POA document into the bank and told them I wanted to put mom's money in a CD which I was able to do without her signature, and that's how I keep the majority of her money from being spent. I also monitor her account online.
I don't know how your parents are mentally, but my Mom can no longer do basic math and needed someone to step in and take care of finances. I pay her bills and just give her some cash to carry around for going out to eat and sundry items.
It seems there's always one sibling in the family who manipulates and takes advantage, since mom thinks she has to help her kids (I know I do a lot for my kids as well), all I can do is try to minimize the damage. Good Luck!

If you're parents are still of sound mind, then I don't know what you can do to stop the 'bad sibling from preying on them'. If on the other hand they are starting to slip brain wise, then putting someone in charge of their finances (POA) is in order. If this has been the status-quot between your parents, and the one kid that is the moocher, then good luck trying to change that dynamic.