Those of us with questions in this section are not doctors, but people living with RA and we answer based on personal experience and whatever knowledge we've managed to pick up.  Some questions are best asked of a doctor and this is one of them - I simply don't know enough about how the medications interact with diabetes to give you any kind of solid advice.  however, I can give you some general information.

Prednisone is only one of many medications used to treat RA and it's a relatively mild treatment. I would recommend you and your mother do some research on DMARDs like methotrexate, Plaquenil, Arava and the Biologics (Enbrel, Humira, Remicade, etc.) and that your mom go back to her rheumatologist to discuss treatment options. It's important to get RA suppressed as soon as possible to protect the joints from permanent damage, but treatment can sometimes be complicated by other conditions, like diabetes.  I'd recommend that your mom speak to her diabetes doctor about connecting with the rheumatologist so they can discuss how best to treat her in a way that keeps the blood sugar regulated.