Sister keeping mom where she doesn't want to stay-applied for guardianship.

If your sister got guardianship then your mom would have had to sign the guardianship papers and the judge at least here in indiana would make sure the person was unable to pass a mental evaluation before allowing the guardianship . They are very hard to reverse so unless you can prove your mom is able to live on her own or you take over the guardianship then there's not much you can do. Good luck, my sister has guardianship of my dad and it's not a fun thing he doesn't like where we put him either even though he's on Medicaid and on the nicest nursing home in the state. He continues to call attorneys to get him out but none will take his case. He has managed to piss us all off with this nonsense now he has no one to visit him anymore. It's sad that people don't plan better for getting old.

Lilacalani,
You are so right!
Further, the elder might appear as perfectly balanced throughout a 2+hour evaluation by social workers....only starting to "lose it" by the time the soc. workers pack up and leave the house.
Mom needs evaluated.
BUT, someone needs to look into what's really going on with the Brother taking her money--she might have given it to him freely, only to carp about it to others, later.
My Mom did exactly as you state--did the classic "divide and conquer".
She utterly destroyed relations between me and my siblings during the 6 years she stayed under our roof.
They still do not seem to understand what hit 'em.
They still think I am furious at them, or furious at Mom, since that has been laced into Mom's stories.
The hardest part about elders who do this, is that they know enough truths, and how to lace them together with lies, to delude even family who know them well, know their habits and behaviors of a lifetime.

It really does sound like Adult Protective Services might need to investigate, too.
If you are at some distance fro Mom, so that it is hard for you to get there, this would be helpful. And it might help put the brakes on whatever hinky things other family members are doing.
We kept reporting abuses/neglect for a family down the street from us.
NO agency responded, not for reported drug dealing in the house where children were, not for child trying to escape over a balcony, not for cold kids w/no sweaters, nothing...the only time agencies responded, was when it was reported that the drug-dealer boyfriend was taking the mom's welfare check, and preventing them getting things they needed to live.
THEN they were all over the case.

So maybe, something needs said about her money going to persons that are not using it to care for her, and her needs are unmet, and she is endangered..
She is. If she is staying at home, not going anywhere, that is a problem. IT might be the tip of the iceberg.

Be sure you get both sides of the story. Your mother might sound competent to you, but to those who are around her a lot, reality might be very different. In my experience, it's not uncommon for the mentally declining parent to turn, or try to turn, siblings against each other.

Your sister has applied for guardianship? Be sure to attend the hearing and have your say!

Brother is stealing from her? Tell the police.

Contact Adult Protective Services to investigate.

I would call Adult Protective Services to investigate.