Nope, I haven't read a single one of the "Thea Sisters"-books, but I've seen enough scans to know that A) They're really cute, and B) they get into trouble on an almost regular basis.
So yeah, I just felt like trying to draw 'em. Just chose two of them more or less at random, mostly based on appearance. Will I do any of the others? I dunno.
For now, enjoy this.