It is a distasteful thought, but if this woman was really a Romanov, she is more likely to have been the daughter of one of the girls than of Alexandra. However, because I do not like to believe the more lurid stories about what happened to the family in captivity, I tend to think she was a fraud (although, in the one picture we have seen, from an extremely dubious source, she does bear a resemblance to Alexandra).
This is not the only rumor about a "fifth daughter of N&A" -- there was a story in James Blair Lovell's biography of AA about a fifth daughter who was raised in the Netherlands. To both of these stories, my response is -- yeah, right.