I probably should have mentioned that short-term visas with working privileges are getting more and more difficult to get. The number issued is limited and the last couple of years, the year's compliment seems to get snapped up within a few days of them coming available.

Your chances are better with an organized programme of some sort. CIEE is probably the best known of the international exchange programmes, but there are others. There are also opportunities to work as a camp counselor (though room and board are usually part of the deal) or some other "recreational" worker - again, the organization gets you the visa.

Or you or your sister could go for an au pair visa - though I'd be extremely careful will that option. The Americans use the terms "au pair" and "nanny" interchangeably and there are no legal distinctions made, as in most European countries, limiting the duties and hours of an "au pair" and requiring that they are enrolled in courses during their stay.
Cheers,
Bev