Hello Mandy,
As you are aware there are two basic types of caravan movers, Those that fit to the front on the A frame, and the others fit to the chassis and drive the main caravan wheels.
The type that fit to the A frame (Mr Shifta is one of them) are fine on hard flat ground, but they begin to struggle if the ground is soft, slippy, or if there is a steep hill to climb (or come down!). I suggest that as a rule of thumb, if one person cannot push or pull the caravan without needing to use the brake, then the A frame movers are likely to have the same difficulty, and don't forget, its not necessarily the ability to push or pull, but more importantly the ability to stop a caravan on a hill.
You also have to store them, and fit them when you want to use them.
I know I have painted a fairly bleak picture, but these are issues you should be aware of.
By contrast, the chassis mounted movers have been reported on frequently by members of this forum 'the best thing since sliced bread' They are available both at home and away, and because of their superior traction, they are more capable of tackling inclines and loose surfaces.
Yes they can get stuck, but most cars would also be stuck under the same conditions.
If you sister only wants it for flat hard surfaces then the a frame are fine, but think about the wider world out there.