Brief Answer:
keep the dose of predmet to least.
Detailed Answer:
Hi,
Thanks for writing in.
I can understand your concern about your wife.
  Your wife probably had a 
viral fever like chickungunya or 
dengue. Both these, especially chickungunya can leave traces of 
joint inflammation for a long time.
  Answering your question, you are right that 
steroids can have side effects over long term usage.
  Many patients get best results only with steroids (like predmet) and these are much safer than pain killers when used judiciously for short term. However, after 2 or 3 months of usage, one may experience weight gain, edema etc that you were predicting. Much longer use can result in 
hypertension, bone loss and 
diabetes also.
  The best option you have now is to keep the dose of predmet to least i.e let her take 4 mg on alternate days rather than daily. If this dose is not achieving therapeutic response, i suggest you to add 
Tramadol which is a much safer drug than other pain killers.
  Keep in mind that these pains can last for months and hence rest to joints is a very important precaution that you must take.
  Hope I have cleared your doubts. 
All the best.