Fanch,
The Cooper S has incredible levels of grip and really rides like if it were on rails, so it is a very safe car indeed (in active safety terms).BUT one of its peculiarities is the behaviour when breaching those admittedly very high limits, its chassis is pretty sensitive to throttle adjustments and is more than happy to flick the tail out instead of just understeering as most fwd would.
This makes it highly entertaining, but for an unexperienced driver it would take a little more time to understand the different dynamics taking place, so my advice would be to choose an engine that wouldn't be able to put the chassis through its paces so much.
In short I would either go for the Cooper, or go for the S with the optional DSC system.
Your sister will love any of the two, I'm sure.They are really well engineered cars.