My sister is currently taking duovir. She wants to change to Duovir-N but it is not available at the moment. Can she use the duovir and nevirapine?

Duovir is the combination of zidovudine and lamivudine, two nucleoside analogues. In most cases, this will not lead to full virologic suppression and the virus will get resistant to one or both of the medications, most likely the lamivudine. In that setting, adding nevirapine is not a good idea, as this would not constitute true triple therapy, but would be adding a single drug to a failing regimen. The virus will likely acquire resistance to nevirapine quite quickly without benefit to your sister. What's more, this will compromise the efficacy of the entire class of non-nucleoside drugs for her in the future. So you see, for a number of reasons, I would not recommend just adding the nevirapine.