Do hernias run in families? If I have one, does it mean my younger brother or my son will also get it?

That is difficult to answer with accuracy. There is probably more folklore than fact in the belief that the tendency to hernias is inherited or transmitted genetically. At this time your groin is normal and there is no way of predicting if and when you or any member of your family will ever get a hernia.

Since many in my family have had hernias, is there anything I can do to avoid getting one? 

The fact is that there is no way of preventing a groin hernia from appearing. Besides which you don't want to and shouldn't limit all physical activity to avoid putting a strain on this supposedly weak area. Do everything you would ordinarily. It is a fact that many people who do heavy lifting in their occupations never get hernias. You don't need to limit yourself to sedentary activities.