Sam's biological dad, Kevin, never makes a direct appearance in his saga, but we know from the multitude of flashbacks that he isn't exactly the nicest guy ever. For one thing, he refused Tia's request to have Sam born at home because "No child of his would be born using what he called 'hippie methods'" (12.11). Actually, Tia describes him as "surprisingly detached from her pregnancy" (12.101), and Sam by extension. Having said all that, it's not exactly shocking that Sam's parents ended up getting divorced.
So what caused Kevin's bad behavior and child abandonment? It's pretty simple, really: He wanted to hide from his family history of necromancy, and he'd do anything to get free of it. While we know from Nick that Kevin "never manifested" the necromancy ability (12.56), it's something he's definitely ashamed of—the fact that he marries Elaine, the poster child for normalcy, in an attempt to keep his children from ending up like Sam or his brother, Nick, is proof of that.
And according to Elaine, Kevin severed contact with Nick after he showed up after the birth of their oldest daughter and said she needed a guide. Kevin's got some serious issues with his past, and unfortunately, he's made some choices that have scarred his son and ex-wife. Time will tell, but we've got a feeling that messing with a bunch of necromancers isn't exactly the best idea, even if they are in your family.