How to prevent brother from taking advantage of mother?

My 85 yo mother lives in her own home w/o any debt. My dad, before he passed, asked me to oversee the money & property he was leaving to my mother & I was made executor of the estate. I am 58, married to a tv director & my father knew my husband as a businessman w/ access to accountants & money managers & my husband could assist me. For 14 years I have helped manage her investments and made sure the property taxes, income tax, etc. everything is paid on time.

In the last 18 mos my mother’s memory is declining. We tried to get her to a geriatrician to be evaluated in Feb & again in April, but she refused to go. She is fearful they will take her drivers license away. I have rescheduled for Dec.
My 1st brother moved in w/ my parents 15 years ago when dad was dying of lung cancer & Brother 1, 57, a veteran, remained in the house to help my mom as my father asked of him. Brother 1 has now been diagnosed w/ terminal pancreatic cancer & is rapidly going downhill & has asked for help w/ mom. She will leave food on the stove, iron on, leave the house w/o her purse or mask, forget what she set out to do, etc. Brother 2, 55, has come through a costly messy divorce, owes me $15000, owes my mom $15000 & owes 10’s of thousands of dollars in credit card debt. My mom told him he could sell his house & move in with her & Bro1, so Bro2 could get on his feet.

He has told both me & my mom the profit on his house sale will be used to pay back each of us the $15000. I believed Bro2 would pay me back until this past week when he told my sister (60), that Mom said she wanted him, Bro2, to be the FPOA and her MPOA. Mom told Bro2 there was $7000 “missing” from her Fidelity acct & she “believes” I gave it to my sister w/o asking. That’s why Mom wants Bro2 to be MPOA & FPOA. I have access to my mother’s investment & banking accts, but no one but my mother can transfer money out. The first time this “$7000 is missing” came to light in April, when my mom saw a $7000 payout on her Fidelity statement. Bro1 called me to say Mom thinks you gave sister $7000. Bro1 had mom call Fidelity & her money manager reassured her no money left her account by my hand, nor could that ever be done, only my mother has that power & the $7000 went to my mother’s AFFCU account. I also called my mother, denied moving her money, that I would never do that & had a 3-way call w/ Fidelity to again reassure her. That happened in April and now this “memory of missing money” is coming back.

My sister has been an ER nurse for 30 years & it has always been assumed she would be my mother’s MPOA. No documents have ever been signed (I was to fly in in April to take my mom to the geriatrician for evaluation & to get the FPOA & MPOA signed & notorized, but because of COVID, I did not travel & Bro1 could not get my mom to the doc appt for evaluation-she refused to go (fear of losing drvs license & what doc may say). I forwarded the docs to Bro2 thinking he could take mom to the notary so I could have the power to hire an at home health aide to be there to take the burden off terminally ill Bro1 to make sure my mom is cared for. Then I find out that Bro2 is stating that Mom told him she wanted him to be both MPOA & FPOA. My sister said there is something wrong here & believes Bro2 is trying to take advantage of Mom & the situation that Bro1 is terminally ill. When Bro1 was told about Bro2’s “power play” he said over my dead body. Bro1 is trying to get MPOA & FPOA with my name and my sister’s names on the docs before Brother 2 attempts to do it w/ his own name.
Any recommendations for what to say to Brother 2 who owes me $15000 & whom I have trusted to act with integrity? And what to do before my mother gives my heavily-in -debt brother the financial power to access to all the money she has in the world? My sister and I are prepared to do whatever it takes to continue to act in my mother’s best interest. Thank you all!
”LuLu”