Personally, from what you've revealed here, it sounds like you're more depressed than Schizoid. Perhaps your so-called 'loner' traits are a result of this depression. You'll find that whilst individuals with SPD are perfectly content, they can and do (at times) develop depression, for varying other reasons, usually because their SPD was a result of trauma or abuse in the first place.

In answer to your original question, I've not yet managed to find anyone in my family tree with SPD. Depression/Bipolar disorder, however, is very common on my mother's side.