Although many families have teenagers who don't get along with their siblings, if they grew up with those family members, one hopes that underneath it all, they do love their siblings. In a case of a step-family, where the years of shared history aren't present, adjusting to new step-siblings is even harder. In addition, while one teenager used to be the eldest in the family, with the accompanying rights and responsibilities, he or she may now be a middle child, yet another adjustment. Another difference in expectations is that the step-parents may be pushing the idea of family togetherness while the teenagers are at an age when they would rather be with their peer group and not with their family.
First, don't assume that everyone will get along with each other. It may take time for the children and teens in the family to appreciate each other's good points and to accept each one's differences. Second, try to give each teen the privacy that teenagers crave. If you can give teens their own room, even if it means fixing up the basement or study, this step can diffuse a lot of the tension, and your teens won't feel they have to share their own room with someone they detest. Third, work on tolerance, where the teens learn that other people can have different ways of doing things that aren't necessarily wrong; and strive for compromise, where the teens try to work out solutions that satisfy the needs of both parties. If you see that communication is a problem, you may want to involve a family therapist who can moderate a family discussion and keep all members focused on productive negotiations.