Thanks to SpoilerFix:

This is the first of at least three episodes in which Sydney's sister will appear.

Thanks to TV Guide Online:

ALIAS SPOILER ALERT!: Stop reading if you don't want a major Alias plot development spoiled... Alias has tapped actress Mia Maestro (Frida) to play Sydney's kid sis — yes, she has a sister — in at least three episodes beginning April 25.

Irina's daughter. And rumor has it Slones the daddy. I'm dying to se how this turns out.

I seond the evil laugh! lets evil laugh together!

This is going to be good..I can feel it...

Sounds like a plan to me!

so thats the sister. Must say, excellent job with the casting.

Yeah...gotta agree..Excellent job with the casting. And from what I've read...she is very, very bad!

Yes especially when they are related to Sydney

There goes that jaw.