I will definitely second the notion that there's no one-size-fits-all answer here, some people fare better than others. It also comes down to how fast you reach that altitude. The slower you take it the better you will fare.

Personally, I do well with altitude and wouldn't give it a second thought. (I live at a bit under 3,000'. Tomorrow I will be hiking over 10,000' with no acclimatization.) Her sister, though--I would worry about her in Lhasa even with taking days to get there.