I am sorry to read about your sister's behavior towards your mum.
Eventhough, there is no formal Landlord-Tenant relationship, your mother and sister are in such a relationship. So, the eviction procedure would be the same as that of Landlord evicting a Tenant who has not complied with the Terms of the agreement, whether oral or written.
Notice to vacate the premises must be given to the Tenant. If the Tenant does not vacate, then the Landlord must file eviction action against the Tenant. See Texas Property Code Title 4 Chapter 24 Section 24.005 and Title 8 Chapter 91 Section 91.001
Here is the eviction process:
There is no self-remedy to eviction actions in Texas. This means that the Landlord cannot lock the Tenant out, shut off the Tenant's utilities in order to force the Tenant out. The Landlord must use the Court system to accomplish the eviction by filing eviction action in Court with the Justice of the Peace Court. Nonetheless, Texas Property Code Title 8 Chapter 92 Section 92.0081 gives the Landlord the authority to change the locks when the Tenant is delinquent. However, the Landlord must give the Tenant the new key. See Texas Property Code Title 8 Chapter 92 Section 92.0081 especially subsection (b)(3) (emphasis added by me) for the proper procedures and make sure that you follow the procedures very carefully. Otherwise, the Tenant may file a lawsuit against you for unlawful outster. See Texas Property Code Title 8 Chapter 92 Sections 92.008, 92.009, and 92.0091:
Goodluck with the case,
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