There are many reasons for this change in behavior. The creation of a new stepfamily means that teens have to give up the fantasy that their biological parents will remarry someday. Some teens also worry that the stepparent will interfere or take away from their relationship with the biological parent. Finally, teens may feel confused by feelings of divided loyalties. If they like the new stepparent, they worry about being disloyal to the other biological parent who may only have weekend visits.
Teens can be jealous of the new marriage and resent it when the couple spends private time together.
This can be particularly difficult if the other biological parent is still alone. It's not uncommon in this situation for teens to feel sorry for that parent and blame the remarried one for causing all the trouble in the marriage and the family. Teens may also express strong negative feelings when the stepparent attempts to discipline them. The situation becomes even more complicated when stepsiblings are part of the picture. In any family sibling rivalry can be intense, but it can be magnified in blended families.