There are many types of arthritis. For now, there are no known cures to any arthritis types. There are lots of effective treatments. The key is to get the correct arthritis diagnosis. That will help establish the best treatment to help with your quality of life and function.
In summary, I highly recommend that she get the disease under control before getting pregnant.
The most important thing is for you to have a great trusting relationship with your rheumatologist. Let them know what you are thinking about. It will be important to discuss if and when you are planning to get pregnant. Methotrexate is associated with birth defects. If you start thinking about conception, you will want to be off methotrexate for at least a few weeks before you start trying. Often but not always the RA will go into a remission during pregnancy. If that occurs you will need little or no medications throughout the pregnancy. However if the RA flares there are still some medications that you can take safely during pregnancy such as low dose prednisone and hydroxychloroquine (plaquenil). You will also want to discuss if you plan to breast feed as that will impact what medications you can take for the RA after you deliver the baby. It will be important to re-start medication post partum because even if the RA goes into remission during the pregnancy, it often flares after the baby is born.
Excessive sweating and drowsiness are not typically associated with sulfasalazine or plaquenil. Those symptoms may be coming from the zomorph (morphine ER) however.