Kat Incorrigible, by Stephanie Burgis

The sisters Stephenson live in their humble vicarage with their father, brother and Stepmama.  Eldest Elissa is soon to be foisted off to old Sir Neville in the name of bringing some much needed money to the family.  Elissa is long suffering and realizes that this is to be her duty and her fate. 
Middle sister Angeline and youngest Kat don't understand why Elissa has to be so good about everything...always doing her duty, never getting into trouble.  Kat constantly finds herself in trouble, as manners and ladylike things are not her forte.  Angeline herself has gotten into a bit of trouble as she has been using her Mama's magic book (strictly forbidden) and has managed to cast a love spell on the unsuspecting Frederick Carlyle.
Mama had been a witch, and it was clearly her downfall.  One of the first things that their stepmama did when she joined the family was to lock away all of the girls' mother's things in a cabinet.  Kat, being the youngest, is insatiably curious about her Mama as she was so young when she passed.  One evening Kat dares to steal the key to unlock the cabinet so that she can know something of her Mama as her sisters do.  If she ever had any questions about her mother's magical abilities they are answered in the darkness.
Before Kat can fully address her realizations and questions about her own magical abilities, she is rounded up with her sisters by her Stepmama to attend a week long house party at Grantham Abbey where Elissa is to meet Sir Neville.  Upon meeting the older gentleman, Kat is overcome with a feeling of darkness.  There is simply no way she can allow her sister to marry this man.  Especially when it is so clear to anyone around her that she actually has feelings for Sir Neville's brother, Mr. Collingwood.
What follows is a wonderful adventure filled with magical orders, intrigue, murder, highwaymen and family loyalty and betrayal.  Kat herself is a fierce and feminist character who relies on herself and takes all kinds of risks rather than succumbing to helplessness.  The pacing is perfect and the cast of characters compliment each other completely.  There is non-stop action and just the right amount of romance.  Kat is someone readers will want to get to know further as they cheer her on.  Readers of The Incorrigible Children of Ashton Place, and The Mysterious Benedict Society will likely adore this one as well.