Black Canary is ok but not Siryn?
Ummm ok. I assume it was no to the flying.
Shall we compare the powers of the two?
Black Canary: at full power, she is capable of shattering metal. Has next to no versatility, she can produce relatively few effects, by using less than full power(i.e. stunning people, make people fall unconscious). That's pretty much the extent of her powers.
Syrin: at full power, she can level a good sized building with little effort. Aside from blunt force screams, she can use her powers to hypnotize/mindcontrol people, she can fly, she can use it to create a sonic shield protecting her against physical damage, she can use it as a sensory power by imitating sonar. And with her vocal cords and throat being tougher than a normal human, she can scream at full power for several hours without tiring.
In terms of 'powers', Syrin is far beyond Black Canary. Black Canary was ok'd because she is within the power limits. Syrin was denied because she is more powerful than Spider-man.