In reply to jonnie3430:
Not exactly what you asked, but crystal do sat to sat or Sunday to Sunday for packages
http://www.crystalski.co.uk/search/sr.2
A couple of the family ski company's do Sun to sun but I'm guessing your sister would sooner stick pins in her eyes than spend a week without her kids but with someone else's (or maybe it's just us).
In terms of booking a hotel/chalet , choose a resort then ask on here for individual recommendations, but good but reasonably priced independent hotels ( of which I reckon someone on here will know for most popular resorts) will probably have been booked up weeks ago. You might get a cancellation .
I've used this website to find apartments
https://www.holidaylettings.co.uk/holidays/france/
Hth
Post edited at 07:39