My niece is 21months & has croup about every 8weeks, has had pneumonia & bronchitis do you think she will be more likely to have asthma or alls?

Maybe, maybe not. Some kids croup easily (with colds) and end up needing treatment for croup (sometimes prednisone) several times. A pediatric ENT doctor may need to check her throat & voice box. Usually, "croupers" outgrow it as they get bigger, and croup doesn't seem to increase their asthma chances. Having repeat "pneumonia" or "bronchitis" is different from croup, and can be signs of asthmatic cough symptoms.
It's possible. She could have underlying allergies or asthma; her parents should discuss her illness history with her doctor. Sometimes seeing a pediatric pulmonologist or allergist may help sort out the problem and create a management plan so she won't have respiratory problems as frequently.