Triple Trouble by Michael J. Pellowski, 1988.

Randi and Sandi Daniels have a cousin named Mandy who is only slightly older than they are and who looks very much like the two of them.  Their two fathers were brothers, and their two mothers were sisters, which is why they look so much alike. When the three of them were young, they used to get along well and they were almost like triplets. However, when Mandy comes from California for a visit, it becomes apparent that she’s changed a lot.

The three of them still look a lot alike, but Mandy has become stuck up and snobbish. More than anything, she wants to be a big Hollywood star. Because she’s an only child, her parents have indulged her, but now they’ve become concerned that she’s become too wrapped up in her ambitions. She doesn’t really have any friends and isn’t interested in anything besides acting. They think that if Mandy spends some time with her cousins, it will encourage her to slow down and act her age more.

But, Mandy’s snobbish attitude rubs Randi and Sandi the wrong way. Mandy doesn’t want to do anything because she might get dirty or break a nail, and she keeps bragging about how grown up she is compared with her not-much-younger cousins. The twins argue with their cousin, and they play tricks on each other. When Mandy takes advantage of acting skills and her similar appearance to the other girls to try spending time with a boy they like by pretending to be them, Randi and Sandi decide it’s the last straw! They decide to show Mandy that playacting is a game that three can play at.

Like the first book in this series, this one is also filled with black-and-white drawings.

This book is currently available through Internet Archive.