From Bing: "Betrayal is a uniquely devastating form of psychological harm. In Dante’s Inferno, betrayers were sent to the Ninth Circle, the lowest level of hell. Those of us who have experienced betrayal understand why."There is a special place in hell for betraying your own blood.
The ninth is the lowest circle. It doesn't specify betraying family, just anyone we are committed to --- but since that's the circle that has Satan standing in the hole grabbing sinners and chewing on them, I suppose the family-betrayers get preference as a sinner-snack.