Only genius can solve within 3 minutes, can you?

Today, I found this riddle online, and it took me a while to figure it out. Now the question is: Can you solve it? Look at the picture below and think carefully.

How many people are in the family?

Check answer bellow:

The right answer is: A) 9

Why are there only 9 people in the family?

Because: six sons + two parents, become eight family members.

Then each son has one sister, but that sister is the same sister for all the brothers, if there were more sisters, then the sons wouldn’t have one sister each, they would have more.