Sister filed for divorce her hoping-soon-to-be ex-husband is being very elusive about his where-abouts. How can she get him to sign papers?

A party can proceed with a divorce even if they cannot find or serve the other party.  When you cannot find the other party, you must file a verified (notarized) motion stating that you have attempted to serve the party, your efforts, and last known address of the other party.  The court will hear the motion and may order publication of the petition for dissolution, meaning they publish it in the newspaper.  The other party has 30 days to respond or appear.  If they do not, the court will proceed with issuing a dissolution of the marriage.  However, the court cannot allocate property without personal service.  Therefore, you would, in practice, likely keep all of the property unless and until the other party shows up to claim some, at which point you could then have the court allocate the division of property if needed.