Quite the opposite really. When serving papers, some people will go to any length to avoid being served, so they need to be really sneaky about getting the papers into the right hands. This could be the case if she's being sued or had caused an accident perhaps.
I'm still thinking this guy is just a wierdo. It seems complicated by your mother, maybe she read the whole thing wrong, heard the wrong title, or something. It's just another layer to get through to figure out what the heck he was doing there.
At the least have your sister call the cops to ask their advice. Also, write down when she called and asked for advice if he does show up again. And, be sure to tell your mom not to talk to strangers
or give out anymore personal advice.