Never partner in business with family or friends. How true is it?

Never partner in business with family or friends. I am in Germany and my cousins are calling me back to India to join family business. My cousins have very successful plywood, laminates and Hardware business in our town. and they are looking forward to establish the business on tier 2 city in India. I have no knowledge about this business, but they are asking me to come back to India and join their business. They earn daily around 10k-20k rupees. During season more than that. Very tempting. They are calling me because they trust me because I have always been very smart and have good communication skills. I am in Germany since last 3 years and learnt German language b2 and applying for jobs here. Shall I move back to India and join the family business or continue for looking for job here?