Bras and Broomsticks by Sarah Mlynowski

Rachel Weinstein, 14, wants many things - she'd love to remain best buds with her newly popular friend Jewel, get a date to the Spring Fling, and stop her dad from marrying her "Soon To Be Step-Monster." When her younger sister, Miri, discovers that she is a witch, Rachel just knows that her dreams are within her grasp. Though the girls' mother warns them that magic can lead to unexpectedly harsh consequences, the sisters secretly come up with plan after plan to stop their father's wedding and help Rachel find popularity.

Cute. Quick, enjoyable read. Funny, fresh, and realistic voice. Looking forward to the sequel.