Is the IIM Indore entrance very difficult to crack?

The level of difficulty of any test or exam depends on your intelligence, IQ, English speaking and writing skills, Mathematics, and logical reasoning. The aptitude test comprises questions based on similar topics. The difficulty varies from individual to individual. For instance, you might be able to ace the aptitude test but falter during the interview ruining your chance of admission. 

My brother was able to crack the Aptitude test as he had been preparing for JEE and the level of Math and English was way below what he had prepared.

The interview is the trickiest part. They can literally ask you anything from the structure of the Indian Government to the weather in your hometown. Most of the questions can be solved with luck and logic. They might also ask you what you have studied so far and what you haven’t. You can never be fully prepared for the interview. Rather than your knowledge they asses your confidence and other soft skills during the interviews.