Welcome to EmpowHER! I am sorry to hear your sister is having such difficulty. She is fortunate to have such a caring sibling and I understand your concern. While symptoms of postpartum psychosis includes paranoia, hallucinations and delusions, the onset normally occurs within 2 weeks after birth. However, from what you are describing, your sister definitely seems to be experiencing mental distress.
She may be suffering from a mental health disorder. Please accompany your sister to her doctor, who can give her a medical evaluation and refer her to a mental health practitioner for a psychiatric evaluation. If you feel she is a threat to herself, or may harm her son, please enourage her to seek medical treatment immediately. In Austin, dial 911 if your sister is in immediate danger, or The 24/7 Crisis Hotline at 512-472-HELP(4357) for psychiatric emergency services.
I hope this answered your question. Please check back and let us know how your sister is doing.