I swear, Doc, there is NO WAY she's pregnant!

This is, unfortunately, a conversation I've had numerous times... usually goes like this...

"I think something is wrong with my (female) dog - she looks like she's getting fat and there's milk dripping from her belly."
"Is she spayed?"
"No, but there's no way she could be pregnant!"
"Has she been around any un-neutered male dogs?"
"No, just her brother."
"Well, I can feel puppies when I push on her belly, and that is in fact milk - she's likely to have these puppies soon."
"Thats impossible! Its her BROTHER - they KNOW not to do that!"