In a word, no.
As cool as it would've been, reports that Julianne Moore is joining the cast of Desperate Housewives as Bree's sister are completely false.
"Julianne is a very talented actress and we would be honored to have her appear on Desperate Housewives" says an ABC Studios rep. "Unfortunately at this time, there are no plans to do so."
Not sure how exactly this rumor got started - particularly since, as far as I know, DH isn't even introducing a sister for Bree. Regardless, it's not happening.