I think Catts are quite easy to grow.......not always so easy to bloom. If you have poor light your sister won't be seeing flowers anytime soon. I think Phals have a real learning curve, but once you figure them out they'll give her years of enjoyment. I think Paphs are very easy to grow and bloom. They like being a bit moister than Phals so if she has a heavy watering hand it won't affect them quite so much.
If you're looking into Phalaenopsis looking type plants to grow that are pretty easy. Instead of plants from the genus Phalaenopsis, why not try something what I consider to be easier to grow, such as Sedirea japonica.