Josephine had become a genie because that's what Genie was imagining when she gave Josephine the genie outfit. And her mental image became real. It was just an accident. She only wanted Josephine to look like a genie, not actually be a genie. But the information given to her by her wish informed her of something else. When Josephine was out of the lamp, she could transform herself back into Joseph and not be a genie anymore, so the situation wasn't all that bleak, for Joseph/Josephine. But in the meantime, Josephine was stuck in her current form. And she couldn't be released from the lamp until someone rubbed it and let her out, along with Genie presumably.
"Well, Mistress? Are you going to tell me what you found out?" Josephine asked, cringing a little. She hated that she was forced to refer to Genie as her mistress. "I can tell by the look on your face that you found something out. What is it?"
Genie thought about whether or not to tell her. After all, if Josephine knew the truth, then the next time they were let out of the lamp, Josephine would become Joseph again, and in turn become Genie's master once again. Why not keep the truth from her instead? It might be kind of nice to have another genie around. Even if the other genie wouldn't be too thrilled about it. What should Genie do?