I wouldn’t get too crazy with this because your sister did lose interest once before, that said, walk into Michaels and pick up a pair of medium sized needles, maybe a 6 or 7 and a pretty color worsted weight yarn. Let her fool around with that before you invest any further. Also, maybe start her off making wash cloths or pot holders, they are such fun to make and come with many, many designs for them. I have been knitting and crocheting for decades, so I do know how costly supplies can be.
Also, what kind of knitting is she interested in, knitting clothes (children’s clothes, shawls, sweaters etc) dolls, toys? The first thing I knitted was a sweater but after knitting two of them got bored with that and gave up knitting altogether. Then one day, maybe 20 years later, I decided I wanted to make a doll for my niece and picked up my knitting needles once more…and I haven’t stopped knitting dolls to this day!! I made over some 30 dolls (so far) and many other toys since I made that first doll. I discovered that I truly enjoy making dolls most and am never bored knitting them as each one is different.
If I had started out making dolls right from the beginning, I may never have stopped and lost all that time when I could’ve been enjoying making more of them. Some knitters just enjoy knitting shawls, or sweaters etc., Your sister will eventually find her own niche…whatever it is that fills her heart when she picks up her needles. That’s the key to a successful knitting experience…enjoying what you do!!