I realize this may be too late, but incase it's not, I thought I'd add something that hasn't been mentioned in the other posts.
I had twins in Sept, and they were very small - 5 1/2 lbs & 4 lbs, 4 oz. They spent almost a month in the hospital & during that time they fell in love with the Fisher-Price Baby Papasan Cradle Swing, Nature's Touch (the hospital had 1). They could swing in it while swaddled.
Unfortunately, it was too much for us to be able to buy 1 - let alone 2 - so eventually we got the Fisher-Price Power Plus Swing. Here's the thing we didn't realize: when we got the swing, they were still too small to sit in it. They hunched over & we had to use head supports until they were about 9-10 lbs.
According to the nurses, most babies love to swing, so chances are they will like to also. I'd be willing to bet your sister will enjoy the ability to clean, shower, sleep, eat dinner with Dad, whatever, while her babies swing safely. The reason we got the Power Plus Swing is not because of the price, but because YOU CAN PLUG IT IN. My boys love to swing so much that we were going through "D" batteries like kids with candy, and it became and additional expense. This swing also has the options to use batteries if they don't end up swinging that much.
I'm not sure what features are most important to you, so I don't know which swing would work out better for your sister, but this was just my experience.
Good luck & congrats to your sister! Twins are truely a blessing!