Actually there is something you can do.
Look under your State Listings for Adult Protection Services. Just like Child Protection Services, they are the agency that checks on Senior Citizens and makes sure that they are not living with or in bad conditions. They will do a "wellness check" (that is both a physical and mental check) on your mom.
If she is being abused (and as you describe it she is, abuse is not always physical) they have the authority to take matters into court, ie get your brother removed, and protect your mom.
Hope that helps.
Love and hugs,