And in response to all the Coffee Roomaratzim, the fact that she is an ervah does not allow yichud.
For example, an eishes ish is also an ervah, and there is an issur yichud. And even if you mean ervah of shear basar, bas achoso is an ervah of shear basar and there is an issur yichud.
I have no idea what the halacha here is, but neither do you.