A great many people who can tolerate exposure to dogs can't manage any time at all with cats before showing allergic reactions. Allergies to cats are so prevalent and so severe that they've been suggested as one of the factors in the increase in asthma, especially since more cats than ever before spend their lives inside. Your sister may well be able to tolerate dogs for a certain period of time, but that doesn't mean she'll be able to live with one. And it certainly doesn't mean she'll be able to tolerate a cat. Any cat, by the way, because there's no such thing as a cat who isn't a potential allergy trigger.
Some people survive well enough with animals and allergies, but it does require some effort.
Keeping animals clean. Keeping animals out of sleeping areas at all times. Limiting exposures to other allergens. Keeping all your allergies under control can help your body handle the exposure to a pet. Most important is to work with an allergist who's willing to work with you.