Very nice portrait. Excellent light and colour...
I think I would have liked it even better without her arm in the bottom left corner. Would have been better if it had been black there as well. Something looks wrong on the edges of both her arms. 7 for now...
I'd give this an 8, if it wasn't that I know who you are, oh great photographer of sisters :) 8 because it is really well done, without being a setup or all gimmicky. Not higher than 8, because even though it is well done, it is still a fairly standard image, and I have to have room at the top, right?
This is a nice picture, of a pretty model... skin looks good to me, and the blakc shirt/background makes her stand out nicely. Her features seem maybe a little flat; perhaps a slight turn of the head or some cross lighting would help?