Fortune tellers generally don't tell fortune to their families.

There is no scientific basis for fortune telling, especially for your grandfather or your close relatives. Even if he really knows fortune telling, he is not allowed to calculate for his closest relatives. Especially in the fortune-telling industry, there is an unwritten rule that people with close relatives are not allowed to tell fortune.

Of course, if you have to ask grandpa to tell your fortune, it is ok. After all, this is only a feudal superstition and will not affect your real life. However, since you are going to tell fortune, you must abide by the rules of fortune telling and give as much money as you want.