I was going to say say exactly what DreamingofSunshine has said. I have had RA for 26 years now,since I was 30,and believe me,if there was a supplement,diet,or medical aid that would help,I'm sure I would have found it by now! My consultant told me,that in her opinion,there are very,very few,if any,supplements,or diets,that make any discernible difference.
Also totally agree re. the makng sure that family and friends know that it's rheumatoid arthritis,not osteoarthritis. The two are completely different. When I was first diagnosed,I was constantly hearing remarks such as 'you're far too young to have arthritis'. Osteoarthritis is usually (but not always) an older person's condition,whereas rheumatoid can even affect young children. It has nothing to do with age. It tends to affect your whole body,in that your muscles hurt and it can make you feel very unwell generally,with fevers,flu like symptoms,loss of appetite and extreme fatigue.
Luckily (if that's the right word),my Dd was 10 when I developed the disease,and she was my only child,so thankfully,she had a certain degree of independence and I didn't have to worry about lifting or carrying a toddler. But believe me,I still found it very hard at times.
I suffered from numerous flare ups for the first 15 years after my diagnosis,but seems to have settled down considerably in the last 10 years. It is vital to get the balance of doing enough exercise and physical activity to keep mobile,but not doing too much and suffering for it. You just have to listen to your body,don't push yourself if it hurts.
And bless you for wanting to help your sister.
I have found the NRAS (National Rheumatoid Arthritis Society) to be very informative and helpful.