I've put peep sights (usually Williams) on many of my lever actions. If your standard open sights are sighted in, one thing you can do is put the peep on before knocking out the rear sight and lining it up with the rear as the rear sight is in line with the front sight. Then tighten the screws on the peep and knock out (or fold down) the rear sight. Take it to the range and shoot... You'll probably have to adjust a little bit but don't be too surprised if you are already dead-on.
I apologize if I'm telling you something you already know.
Good luck to your sister!
