Julia Murney already played Jenna's half-sister on 30 ROCK, in a very funny parody of an Oprah-type talk show.
I pretty much agree with Gaveston. McArdle has no feel for comedy, as was evidenced in her terminally unfunny performance as Penny in URINETOWN. Krakowski is deliriously funny on 30 ROCK and her humor has served her well in certain stage roles. McArdle's voice, to my ear, hasn't aged as well as Krakowski's.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body