Now that Dawn and Mary Anne are friends and sisters, Dawn wants them to do everything together — share a room, talk all night long, wear each other's clothes.
But living with Mary Anne isn't exactly what Dawn expected. Mary Anne brags about having a date to the school dance, her kitten throws up on the rug, and she hogs Dawn's baby-sitting jobs!
Dawn's always wanted a sister. Instead, she got Mary Anne, the wicked stepsister of Stoneybrook!
Dawn's Mom and Mary Anne's Dad have just gotten married and Mary Anne catches the flowers thrown by Sharon Porter, Dawn's mom.
Dawn is extremely envious of Mary Anne for catching the flowers instead of Dawn, thinking that she should have because it was her mother's wedding. In fact, Dawn says that she would have gladly killed to have her hands on the flowers and the she and Mary Anne are only 13. But soon Dawn calls Mary Anne sis and things are fine, for a while.....
Later, Dawn and Mary Anne struggle to be best friends when they have trouble getting along. When Mary Anne's dad Richard makes breakfast he serves Sharon bacon knowing Sharon can't stand bacon; but she says she likes whatever he feeds her for breakfast. Then again, at dinner the same problem goes on when Sharon makes tofu and Mary Anne and Richard do not eat tofu.
Richard also declares spring cleaning while he is very organized and Sharon is a scatterbrain, so Mary Anne picks up after Sharon, which only irritates Dawn even more. To drive Richard crazy as revenge, Dawn messes up his sock drawer, however this scheme fails, because Richard thinks it was his fault for the mishap.
Dawn and Mary Anne are furious with one another after the events continue in the Schafer-Spier household: Sharon is disturbed about Tigger, Mary Anne's cat, throwing up on the oriental rug. Therefore Mary Anne starts spending most of her time at Kristy's house during the family fight but does not bother Dawn.
Finally, Dawn pretends to be old Jared Mullray (the ghost that haunts the secret passage) to scare Mary Anne into switching rooms. Her plan works and they are finally friends again.
Dawn decides to tell everyone in her family how she feels about the family and thinks everyone should stop trying to impress everyone and to be honest with each other. Sharon suggests that Mary Anne and Richard make food they like on Sunday and that she and Dawn they like on Sunday. That way they both have foods they like. Everyone agrees.
- Meanwhile, the "Pike plague" is going on at the Pikes when the whole family gets sick including Mr. and Mrs. Pike. Claire (5) and Margo (7) get bronchitis, Nicky (8) re-breaks his finger, Vanessa (9) sprains her ankle, Adam, Bryon and Jordan (triplets-10) get pneumonia, Mallory (11) has the Chicken Pox, Mrs. Pike injures her knee while playing tennis, and Mr. Pike burns his hand while cooking dinner.
Ann M. Martin's Dear Reader Letter
In Dawn’s Wicked Stepsister, Dawn acquires a sister when she’s thirteen years old. My sister, Jane, came along when I was two, so we grew up together. While we never shared a room like Dawn and Mary Anne tried to do, we had plenty of fights. For instance, long car trips frequently ended up with the backseat divided into two, and shrieks of “She crossed over the line!” or “She’s bothering me!” or “She touched my stuff!” When we were very young, I thought Jane was a big pest. I was always concerned that she would snoop around my room, which led me to booby-trap my bookshelves. Of course, she never set off the trap. I guess all sisters have fights sometimes, but like Dawn and Mary Anne, Jane and I eventually wound up as best friends.