Nevinny doesn't like telling people about her devil fruit power because she has an awful past of being used by people to make them rich (via turning things to gold). She was even used by her own father. For that reason, she didn't tell the Red Eyed Pirates about it until CP11 told them, and even when Toshin and the gang went to rescue her, she thought they were still trying to get her back to use her until she realized Toshin wasn't that kind of guy.