If even the most powerful Forsaken are careful around Semirhage, then what does this say about their relation to Alivia?
Two things... One is that when Alivia vs. Nynaeve is considered, Nynaeve wasn't at peak strength (at least, we had no such indication). It was Reanne who made the comparison. Does Reanne even know how to measure a woman's potential? It could well be that her comparison was between Alivia and Nynaeve's current strength. And I think everyone only argues that Nynaeve's potential matches Semirhage's.
Second, wariness around Semi means nothing. LTT was weary of her, Graendal thinks well of her, Mesaana and Asmo have kittens over her. These people occupy different strength levels, but sho healthy respect for Semi.
It could be that her legendary torturing skills, as well as her general skill at channeling, are what make the other FS regard her as they do.

Despite my earlier comment about Nynaeve and Alivia not being all that far apart, I do think Alivia is of the same strength as Lanfear.
Thing is, this Age is filled with mavericks who match and exceed the Second Agers in some things. We have to believe that Logain and Taim are merely a slight bit weaker than Rand, putting them in the Sammael-Demandred range. If we can believe that, if we can believe in the existence of Rand, who matches the strongest of the Second Age, why not a woman of Lanfear's strength? Especially since she is not any woman, but one whose destiny is bound with the Dragon Reborn's in a critical way?
Moreover, even if Alivia were only as strong as Cyndane, Nynaeve can still be as strong as Semirhage and Mesaana/ Semirhage and Graendal.

Sorry, but no go. With the number of men populating the very highest strengths on the light side, I find it exceedingly hard to belive that the strongest Lightsider woman is merely as strong as the second or third strongest Female Forsaken.

Nynaeve is not quaking in her boots about Semirhage like she used to about Alivia. She doesn't even seem that uncomfortable around Alivia in this book or KoD. I think, for thematic, as well as practical reasons, Nynaeve matches Semirhage, who is herself one of the most powerful Forsaken.