The kids in Holes are totally removed from their families, and for the most part, we don't really see them in the context of their lives at home. The environment at camp sure doesn't make for a feeling of family either. But the fact that family is lacking is a really important aspect to the story: sometimes what's missing is just as important as what's there. The boys, having been separated from their families at camp, are vulnerable to the power-hungry adults (and kids) around them. Zero, who hasn't had a family for quite a while, shows us how this vulnerability can happen anywhere – not just at Camp Green Lake. Bottom line: without family, life just isn't as cozy.
Stanley's relationship with his family is ultimately the best thing in his life. It's what keeps him going despite all the bad luck he has to endure because of the curse.
Friendship – particularly Stanley's friendship with Zero – is way more important than family in this book.