My sister did a gastrectomy or gastric sleeve surgery, now after a month, she can't eat anything even if it is one bite of an apple. is that normal ?

No: It's possible she's developing a stricture or narrowing. I recommend she sees her surgeon as soon as possible. Sometimes an upper endoscopy with dilation is all that is needed.

Not normal: That is not normal and you should be able to eat about a palm sized portion of food, although it may take time. Protein and hydration are very important after bariatric surgery. If she cannot tolerate even a few bites of food, she needs to see her bariatric surgeon ASAP! She could have a stricture which would need further treatment.