Well here is my advice, and I know it won't be popular.
First some questions.
Do you care about your nephew?
Are the two of you close?
If the answer is no, then just leave it alone. The only meaning it has under these circumstances is puerile family gossip.
However, if you are close and you do care, here is my advice.
Book another session with her strictly to talk to her about the situation.
She is most likely a reasonable person.
Explain to her that you have no intention of interfering, but you care deeply about your nephew, and his well being.
Ask her if he is aware of what she really does. Which he may very well be.
If she says he isn't, explain to her that keeping this from him is wrong, and can be much worse the longer the lie continues.
Tell her your insights on how he might take the truth, and even how she might go about telling him.
Offer her your insights on the situation and make her understand that you appreciate the difficult situation she is in.
Re-iterate that you will not tell him yourself under any circumstances, regardless of what she decides to do, but because you care about your family, you wanted to help her overcome a potential speed bump that can be difficult for both him and her.
This way you can be assured of what her intentions are, and guide her in not hurting your nephew, which I'm sure she is desperately trying to figure out on her own.
And if it turns out he already does know, then you can breath a sigh of relief knowing he is walking into this situation with his eyes wide open, with a girl that is being honest with him right from the start.
