One option is to request someone be appointed guardian for your mother. It is a last resort for a person in need of care as a guardianship takes away a person's right to make decisions for herself. Here is a quick summary of guardianship https://texaslawhelp.org/article/guardianship but note that you cannot file for guardianship on your own. You must be represented by an attorney. If you can't afford one but believe your brother is abusing your mom financially, you can file a report with Adult Protective Services or you can request the court in your county initiate a guardianship investigation (this is an example of a letter requesting the court get involved https://www.traviscountytx.gov/images/probate/Docs/court_initiated_guardianship.pdf). A guide on guardianship from the state. You can also file a police report and request the District Attorney help with a temporary restraining order until your mom's safety is assured.
On the other side however, Texas has no forced heirship law that says your mom has to leave property to you. If she has the capacity to know what she's doing, she can make any gift she wants and you have no right to receive her money until after her death.