Your kids and your spouse's might suddenly have become family, but it takes time and work to make them feel like family. Start with brief, well-planned get-togethers that are designed to be fun for everyone and build up to spending more unstructured time together.
Be open about the challenges that face you as a newly blended family. Talk to your older kids about the kinds of adjustments that they'll have to make. Teens are old enough to understand the issues at an intellectual level, even if they still have emotional difficulty dealing with the situation.
Strive to balance between care and love for your biological children and making a positive relationship with your step-children. At the beginning there are sure to be claims of favoritism from one side or the other (or even both). Talk over the problem with everyone together, and let your kids know that you're working toward equality for them all, although it may take time to get there. It may be particularly hard if one set of children moves into the house that the others were already in. New traditions and a feeling that "we're all in this together" can help build a sense of sharing. Point out what has been gained by blending families.
Discuss rules and expectations with your spouse so that you're both on the same page. Explain to your teens that there can't be different sets of rules for different kids living in the same house, so there may be some changes in what they've been used to. Involve your kids in setting realistic boundaries.
You can't force your teens to like their new step-siblings, but you can expect them to behave civilly to one another and to respect one another's feelings. Privacy is important for teens. Shared experiences are vital to building a good relationship, but make sure each child has the ability to spend some time alone when needed.