By the way, all those famous photos of babies with their heads on their arms or doing other cute poses: the babies were posed that way by the photographer. Babies by themselves just sit there.
Newborns? If only that were true. If they're not sleeping, they're crying...
Or pooping. Lots of pooping.
Seriously though, as far as lenses go, the 85mm ought to be your best bet. But, like others have asked, what exactly are the conditions you are shooting under and what are you trying to do? You kind of sound like you are freaking out over this.
If your sister wants you to take baby pictures for an album, announcement etc., it's reasonable to expect her to set aside a half hour to an hour minimum for you, her, the dad and the babies â€“ alone without anyone else interfering. If she's not willing to do that, then she must not want the pictures very badly.
Lots and lots of questions...are you in a warm or cold climate? (warm climate means you can take the baby outside and lay it on a blanket in open shade for nice long shots that could use the 300mm. I did something similar for a grandson born in July and a granddaughter that was born in November -- when we had a warm day. Obviously wouldn't have worked in January in the Midwest) You can get good shots with Mom holding baby (Mom looking down adoringly at baby, baby probably asleep, Mom probably standing near a window so light coming in from the side, close crop of Mom's profile and baby's head and upper torso)
Little hands and feet always good. You can see a few examples of my grandkids here. http://www.canonrumors.com/forum/index.php/topic,2427.msg51769.html#msg51769
Since you are dealing with twins, you'll probably want some shots with Mom and Dad both holding one each. 85mm may be a little long for group shots of all four, but perhaps not.
One shot I would definitely do would be to lay them down on the floor, one going one direction and the other going the other, with their heads next to each other and then get on a chair and shoot from above.
Getting down to their level and shooting from above them will help move your pictures out of the standard snapshot category.
I wouldn't rule out the 580EXII. Best if you can use it off camera to simulate natural light, but even bouncing it off a ceiling or wall can be good.
I can't emphasize enough though that if they want good pictures of the babies, mom and dad have to cooperate and give you the time and seclusion to do it right.