Is it okay to have my niece do capoeira if she has juvenile rheumatoid arthritis?

Yes, but see doctor: All exercise is good for preserving joint motion. The danger would be if there were to be strong physical contact with a damaged joint that had lost range of motion. In juvenile rheumatoid arthritis, some joints become immobile and cannot bend normally. Also, the cervical spine should be examined for prior damage and the bone density should be assessed for osteoporosis. Needs doctor clearance.