As you said, when there are more people in the house your mother is insecure. You need to see what is the reason behind it. Is there anxiety related to it or she gets paranoid/ suspicious (that they will harm me or they are talked na about me) about people .
Diagnosis is important in this case and medicines will help her to reduce these symptoms.
Next Steps
Psychiatrist opinion and treatment.
Health Tips
Accept her illness and don't blame her.
Help her to come out of this problem by taking treatment.