Hi, I'm Dr. Jill. Because of their diet, ruminants like sheep normally have alkaline urine. So I would not be concerned about a urine pH of 9.0...it's relatively normal. As for the arthritis, I presume this has been a progressive ongoing issue for a while? Is she still able to get up and down and walk around on her own? Any secondary issues such as foot infection, swellings on legs due to injury, cuts/scrapes, etc?
For arthritis, maintaining movement and decreasing inflammation are some of the main management techniques. Aspirin is a medication that can be given to sheep to help with pain and inflammation. The maximum dose for sheep is 100mg per kg of body weight by mouth twice a day, though I wouldn't recommend using the full dose long term. You may want to try a full dose for a couple days to see if it helps and decrease the amount or give it intermittently to maintain her comfort level long term if it seems to help.