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I am sorry to hear you are worried about Sierra. My name is Rebecca. I am a veterinarian and will do my best to help.
There are many reasons for an older cat to lose weight, and not eat well: Heart disease; kidney disease; cancer (sorry to say); pancreatitis; liver disease; infection; hormonal disease (hyperthyroidism is common in older cats). Some of these conditions can be treated and managed with medication. If I could, I would start with a thorough physical exam: Palpate internal organs and lymph nodes, listen to heart and lungs, feel pulses, take her temp with a thermometer, take her blood pressure, etc. Some basic blood tests would probably be recommended as well.
Until you can get her checked out try to keep her hydrated. If she will drink milk, also try tuna juice, chicken broth, or adding some canned cat food to the milk. I would not recommend force feeding her until you learn what is wrong with her; this can be stressful for cats and can sometimes cause them to vomit.
Check her gums; are they pale or pink in appearance?
Let me know if this is helpful and what else I can help with or answer.
I hope you were able to see what I sent. How is she doing? Can I help with or answer anything else?
How is Sierra doing?