He may only be a recent abandonee. There are a couple of kitties in my area that are strays but most definately had a home at one time. They are waery of people but you can get pretty close before they bolt. If he was in poor condition, dirty & hungry I wouldn't bother with the fliers I would just keep him or let your sister take him if that is what she wants. On the other hand if is is fairly clean and not skinny and not too hungry then I woudl put up the fliers just in case he is actually a lost kitty. Good Luck and let us know what happens.