If I help a family member after a detox program by co-signing a car loan for them and they relapse and hurt someone while driving, am I liable?

You might be liable. Normally, a co-signor is not--he or she is liable on the loan, but not for tortious acts (e.g. accidents) of the car's owner. However, if this person poses a more-than-usual risk, which they presumably do, since they clearly had a substance abuse problem, and if the only reason they could get a car was with someone to cosign, and the cosignor knows of their substance abuse problem, then it's possible that it  would be concluded that you yourself acted negligently in helping to put a person with a higher than normal risk of accident on the road. This could potentially lead to you being considered liable.