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I am sorry to hear about this situation. No, this is not true. What you are is a tenant at sufferance. This means a tenant who is staying without payment at will of the owner. But while this is the weakest form of tenancy and the owner (mother) can ask one to leave at any time, the owner owes the tenant the same rights to respecting their things like a regular tenancy. This means she cannot simply pilfer what she wants. In addition, common law civil doctrine disallows people to do this.
Technically, one can tell her to stop, and if she does not, to sue her in court for conversion and trespass to chattel. Conversion is civil theft. Trespass to chattel is interfering with ownership of items.
However, if this is done, she can easily then ask you to leave since again - as tenant at sufferance, you have no right to stay there. So with a few days notice, she can ask you to leave and then evict you if you do not. Whether or not she would do so if you threaten/file suit on her is her decision, so she can "retaliate" by asking you to leave.
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