Siblings are not allowed to serve on the same ship; I assume that extends to the same unit too. This policy is detailed in "Left to Die: The Tragedy of the USS Juneau" (great read, BTW). That's the boat the five Sullivan brothers died on and also the four Rogers brothers. I dunno if the other services have a similar rule. The book, obviously, doesn't talk about married folks. I don't think the rule would be the same for other than siblings as the intent of the rule is to protect a family from losing all its children in one event.
I know 2 brothers have been in the same guard unit for some time now. They both fly for the 192nd out of VA.
Actually, I can think of more than a few guard units where they have siblings or the father/son relationship going on within the squadron. I'm pretty sure a few of them have been on the same combat deployment as well. (Though in the guard it's only 30-45 days, they split up a 90 day deployment between 2 or 3 units)