QUOTE (krodgers @ Oct 5 2009, 03:39 PM)

No kiddin! Is Mark the brother of Angie Harmon?
It's hard to say since there's misinformation about him all over. eg. he's NOT the grandson or great-grandson or whatever it was of a former SecNav. His mother is Elyse Knox but Knox was what she changed her name to (and why everyone keeps claiming she's the daughter of former SecNav Frank Knox), her birth name is Elsie Lillian Kornbrath.
Back to your original question, from what I can gather, he's got two sisters. Kriston and Kelly. Kriston was married to Ricky Nelson and Kelly was married to John DeLorean (yeah, the coke car guy).
Some other trivia, he supposedly dated Karen Carpenter. Remember the episode last season where he went back home? They led it to where he met his first wife, at the train station and where the "Rules" came from or at lest the concept for his rules. When the two of them were sitting waiting for the train or bus or whatever it was, the kid that was the young Gibbs, is actually Mark and Pam's oldest son.