It really doesn't. You don't need keyboard or massive case. You have to sit up straight and use your lap/table for a laptop. Tablet you just hold.
the touchpad is heavy imo, i would prefer to have something in my lap than be holding something up, and a proper keyboard makes typing, etc 100 times faster.
it depends what your going to be using it for but in 9 cases out of 10, a laptop is better, they have much more cpu power and ram for a start as well as applications, etc.
i also find the wifi on the touchpad to be slow, on my pc pages load instantly, on the touchpad they take several seconds.
what will your sister be using it for, you said watching movies and browsing.
a tv is far better for watching movies on, and if i was on the go, it's practically impossible to watch a movie unless your on a 2 hour train/plane journey.
watching a movie in 20 minute clips would be horrible, i'd rather just listen to music and watch the movie when i get home.
same goes for browsing, with the touchpad you can only browse indoors, and for browsing not having a keyboard makes it a lumber some task.
tablets are fairly crap imo, it's a luxury gadget which fills a hole in the market which didn't really need filling, if you have money to blow then sure it's great.
my cousin plays poker on his ipad a lot, and guess what he hates holding it up, he prefers to have it lying down, in which case a laptop is so much better, bigger screen, better sound, etc.
i have a touchpad at £100 sure its a no brainer, talk about spending over £250 then a laptop any day.