A cardboard box won't work, because you need something that diffuses the light, not blocks it.
Cheap way you say? That I can do, I'm all about the cheap.
Take that cardboard box with 1 side open, then cut most of the sides and top out.. like if you were making a cardboard tv on each side

. Then replace that space with thin wax paper and put lights on both sides (table lamps would do, the brighter and more like daylight the better). That way you can use bright lights that won't directly cause shadows on your models. Then pin up a sheet on the bottom top part and let it curve toward the front bottom. Tell your sis she should invest in a small light box and some daylight side lights

(they can be as cheap as 50 bucks)
I hope my description makes since,
lol.
I shoot with a Canon T1i, but as any photographer will tell you, the best thing to shoot is great light. Great light can make even iPhone images look pro.
Good luck man!
Edit: Forgot to say, unless she has a mounted flash, try to avoid it. Try to get it bright enough in that little box so you don't have to use it.