Crow and Sirene really do not get along and Crow is deeply suspicious about the fact that 'Sirene' is there, given that he knows perfectly well that Setanta would never have sent his sister... and, yeah, he's perfectly right on that.
He's also been crushing on the real Fel for about six years after meeting her once as kids, so he figures out real fast what the deal with 'Sirene' is (he does not, however, mention this fact to her until much, much later in the novel series). His attitude towards her as 'Sirene' softens very quickly because of it. The first impression isn't the greatest, but he's actually a fantastic love interest and the later interactions with Fel (in both her disguises) are adorable.