My daughter left her husband and kids for a guy she met on the net, she has been slandering her entire family and making threats and accusations all over town what can I do

How was she slandering? What was she saying and how?

Slander accusations (along with defamation of character) suits are very hard to hold up in court mainly because you have to show evidence that it has affected you in such a drastic measure that it has caused you financial hardship. Granted, it may be worth a try to speak to a lawyer about this matter, but very many lawyers are not willing to take the case to court, and not many judges are willing to listen to it either.