We have two cats and neither wanted to be near the baby for the first few weeks so the problem pretty much took care of itself. Babies make weird noises and move in unpredicatable ways. We also act "strange" around them and cats don't like any thing to change, ever. They eventually warmed up to us again.
Despite the recommendations of Dr. Sears, et. al., we never banished them from the bedroom. Figured we weren't the best housekeepers anyway, so it was unlikely the allergen level would be appreciably lower had we chosen banishment. Our little cat formerly slept in my armpit and the big one at our feet (their choices). The little cat moved to my husband's armpit and the other eventually came back to sleep at our feet. Ever since dd learned to get out of bed by herself we've had a gate on the door at night that only the little cat can jump, so we're down to one cat now, and miss the other.
That said, I would never put a baby in another room in an effort not to hurt the cats' feelings. [Having infants in your room in and of itself dramatically reduces the chance of SIDS.] I suspect your sister wouldn't either once the baby is no longer a hypothetical. Wouldn't help anyway... the cat would still be upset by the change in overall routine; why would this one thing out of all the others make such a big difference?
I'm a cat person... love them to death... but they've got small brains, they can adapt. If your sister and her SO manage to feed the cat on time and change the litter at acceptable intervals, they'll be miles ahead of most of us during the first few months.
Just make sure the cat gets indivdual attention every day... brush it if it likes to be brushed, rub its ears, whatever.
One thing they can do it put as much baby stuff out as possible before the baby arrives so the cat has time to adapt to (and mark) some of the physical changes in the environment. I've also heard that if the baby is born outside the home it's helpful to bring an article of clothing that smells of the baby home for the pets to sniff.