ADHD in multiple children?

I have a 5 year old just diagnosed with ADHD, my 13 year old has all the signs of Inattentive ADHD and my 11 year old shows many, many signs as well. What are the chances three of four children could actally have it? Is that normal? I was diagnosed at 14 and on Ritilin for a year or so and then just stopped and am on no meds currently.

[Sandman2]

In, "The ADD/ ADHD Answer book," by Susan Ashley, she says (p 13) that since  about 50% of parents with AD/HD  have a child with this disorder  - " there is strong evidence that AD/HD may be inherited".  Good book by the way.