There are many different series within the Geronimo Stilton franchise, and this subset seems aimed at readers who are intrigued by celebrity stories and fashion. The Thea Sisters have a little more friend drama occurring, and fewer goofy, humorous incidents involving gross things. The story moves quickly, and the characters are not well developed, so it helps that each of the girls has a particular interest (violin, sports, cooking, etc.).
The real draw of these books is the use of color and engaging fonts on each page, along with the full color illustrations. Everything turns out well in the end, and the Thea Sisters claim Lane as one of their own, telling her that "true friends are forever".
Readers who have outgrown Babymouse and want a series with a lot of books will do well to check out this branch of the Geronimo Stilton series.