What can I do about financial abuse by sibling who's constantly borrowing large sums of money?

She's not incompetent but does have a mental
Disorder that could be argued in court as being not capable to make financial decisions.

This is a real loan as far as I know but what I don't like is that it's a $100k loan unsecured where as her money is insured in the bank. My sister is the POA and she does whatever my brother wants basically. Plus I don't like the fact that I had to find out randomly from my mom and it wasn't mentioned to the rest of the siblings.

Is it truly a loan, with payback and possibly interest? Or is it a gift.

If it is gifting, that will mess up her chances for Medicaid down the line. Perhaps you can talk to her from that point of view. That she needs that money for her own care and he should be able to provide for his own needs.

How does she access this money? Does she write him a check? Does she have to call the stockbroker? That's a lot of money to just hand over to someone. If you have POA, you might consider a bank account or trust where she doesn't have free and easy access to the money. I would definitely consult an attorney who specializes in eldercare finances and let Mom pay for it out of her funds.

I would consult an attorney ASAP, as it seems as if your brother is increasing the amount of money each time, and that alone seems suspicious. Also, you may want to check to make sure he has not had your mother change a beneficiary, will, etc. to favor him.

If your mother has not been declared incompetent, then it is going to look like she made an unwise decision. Does your mother have dementia or Alzheimer's?

I'd check with a lawyer to see if anything could be done. I think that you are going to have to look into getting guardianship for her to protect her form herself financially.