I know that in kid's lit, it's tough to have adventuring be a family affair, especially for the parents. As someone pointed out at ReaderCon this year, "You don't take Mom on an adventure." (Sad to hear, but true.) This is why we so often see kid characters as orphans, or minus one parent (usually mom), or that parents are clueless, neglectful, or otherwise absent--it not only gains us instant sympathy-points, but it makes it easier to explain how a minor can get into major trouble. But as is often the case when taking the road most traveled, it's important not to throw the baby out with the bathwater and the devil's in the details. (For these and other fun clichés, please see earlier posts about pet peeves and red pens.)
The real issue for me is the danger of characters have been created in a vacuum, full and complete all by themselves, as three-dimensional as cardstock. Ask anyone who has taken Psychology 101 or ever lived in any kind of family ever: kids reflect their parents, and siblings, and vice versa. Whether you believe in nature or nurture, people grow like bonsai trees: in response to their influences, whether in alignment or in opposition. This includes (but is not limited to) the good, the bad, the ugly, the humor, the quirks, the mannerisms, the pet peeves, whatever. Limiting a character by having the physical characteristics in common with their family members is the same as describing a character only using hair and eye color or dangling a younger sibling (or parent) out the window to tempt the oldest sibling into action: it's commonly accepted storytelling, but it's thin. There's a whole lot more to us than the sum of these parts.
Think of characters that are part of a family (less common than one might think): How are they the same as their siblings and parents? How are they different? Where do their speech patterns or sense of humor or emotional outbursts mirror one another, feed off one another, push one another's buttons or grew up in complete contrast to these influences? What Weasley brothers sounded alike and which ones sounded completely different? Why do you think that is? How was Meg different from Charles Wallace? But what did they have most in common? How about Peter, Susan, Edmund, and Lucy?
Siblings are pretty easy: Cassel Sharpe and his brothers or sisters Rose and Scarlett: you can see their shared pasts and commonalities reflected in their actions and attitudes now because they are present and accounted for. But when you remove parents from the scene for whatever reason, it's important to remember that all these lingering influences remain, even if they were only around for a short while, even if they left or were removed when the character was very small--being gone is a pretty big influence on the personality of a child! Even then, an orphan left on a doorstep, can grow up reminding people who knew the family of their parentage; maybe a sort of gesture, the way the voice sounds, a facial expression, a temper, the way that they walk, all of these are hints to the origins of the character. Don't short-change your orphan by thinking that there's no yearning to know and emulate (or defy) where they came from. Think of Harry Potter or Artemis Fowl or Damien Locke or Batman. Who were each of these kids in light of their parents? How much of their personalities and life goals were drastically affected by these people who aren't even on the page?
Where we come from is as much who we are as where we're going.
Think about it. Your story will thank you.