ADHD has a definite genetic link. However, I have never seen any thing that gives the odds of it being directly passed on. I would think that the chances go down if neither your nor your husband has it. But with both your brother and sister having it, the chances do go back up. But I would think that it would be better than 1 in 3 for your child not to have it.
If you were going to get it, you would most probably have had symptoms by now. Don't fret, and enjoy your good fortune.
My brother has adhd, and my sister has add .. If that helps?