I also saw a copy of this book in my local Barnes and Noble last week. It was sitting on a shelf in the young readers section, which I believe is for ages 7-12. I am not sure I would agree with the 10+ age range, but I can see it appealing to younger readers. I would be more likely to call this an intermediate (grades 3-5) read than a middle grades read (grades 6-8).
The story in The Silver Rings is straight forward. It unfolds like a true fairy tale. Readers will enjoy spotting old favorites that have found themselves twisted up in this plot. Also, like true fairy tales, there isn't a lot of character development. The sisters are very close--connected because they are twins, and because they have magical rings. The sisters are not, however, profoundly deep. None of the characters are for that matter. I don't think they really bothered me because I knew this was a fractured fairy tale and I wasn't expecting a super detailed story in only 223 pages. But, that simple fact is what makes me consider this book for younger readers (intermediate level). I do not think older readers will be happy with the quick plot and sometimes random/rushed actions by the sisters.
Overall, I thought this was cute. It would be perfect for a little girl looking for her first "big" chapter book. The pictures help illustrate what is going on in the plot, and the chapters aren't too long for a younger reader. I enjoyed it.