1x22 - Family Affair

Buffy gets chosen for a lead in a ballet, which is totally annoying for a mother whose little girl, Melissa, was passed over for a complete stranger. She has reason to suspect foul play when the casting director is Bill's old girlfriend Margo, but she knew nothing about Bill's children until after she had cast the roles. Then Margo tells Buffy that she will be a big success and travel all over the world. Buffy gets the idea that it means right away, so she botches up her role in the production so that she will not be separated from her new family.