Why did Percy get such a kick out of calling Vex "sister" when they were all Seeming-ed up to look alike?

Because he's a dirty little fuck, of course. I'd love to any version of Percy thinking two is better than one, whether it's him and both the twins, him fantasising about the twins with each other, him convincing Vex to go for it while he still looks like Vax, or whatever else you can come up with.