A Family Matter

So, as previously mentioned, I've been watching The Sopranos, and it occurs to me to wonder if perhaps someone out there can answer a question that has been driving me absolutely crazy about this show. Namely: How is Tony related to Christopher? I know Chris is his nephew, but he certainly doesn't seem to be either of his sisters' kid, and I can't remember that Carmella even has any siblings. Anyone? I could try googling, but a) I'm not entirely sure what to search for, and b) I'm only in the middle of season 3, and would prefer to avoid stumbling across spoilers by accident. But, honestly, this is the sort of thing that I lie awake at night and wonder about.