You don't. Unless the kids in question are all under age 3 you are NOT NOT NOT their parent. You are their parent's spouse END OF STORY. You do not discipline, you do not set rules. You can discuss rules in private with your spouse but the children should never know they come from you.
The only time you may step in is if you are the only adult present and they are about to do something dangerous. Otherwise it can wait until their real parent is home to deal with it.
Almost every child psycholoigst, therapist, etc agrees with this.
http://www.fortnet.org/ParentToParent/PFellers/par_step.html
Advice For Step-Parents
* Education yourself on what being a step-parent means; it is different from the role of a biological parent. Join a support group or read books on the topic. Recommended is "Step-Family Realities" by Margaret Newman. Contact the Step-Family Association of America, out of Lincoln, Nebraska. Step-parents themselves, the group offers a newsletter, catalogue of books, conferences and on-call counseling. Call 402-477-STEP.
* Work on your relationship with your spouse -- for the couple is the architect of the family. Discuss expectations and problems that arise.
* Introduce children to a possible mate gradually during dating. After marriage, realize that it will take time (from 18 months to two years) for children and adults to get acquainted and adjust. After all, it was the adults who fell in love with each other, not the kids.
* Be flexible and understanding in your expectations, demands and time. Situations can be even more complicated with two sets of step-parents, step-children, and half-brothers and sisters. Biological parents and children need time together, as well as the newly formed family unit. Responding to individual needs is even more important when mates bring adolescent children to a new union. The teenager's goal is for autonomy, at the same time that a new family requires greater togetherness to bond.
* Make your mate's job as step-parent as easy as possible. Remind children that he/she should be treated with respect. Remember that the biological mate has the primary responsibility. A step-parent shouldn't be a co-equal with the biological parent in terms of discipline; this could be a set-up for failure. But the step-parent can be empowered to deal with situations when necessary. Use a family conference to establish house rules and avoid power struggles. The babysitter model works well in terms of discipline. The biological mate can state to children, "Your step-dad will be in charge while I'm gone". And the step-parent can state, "This is the house rule." or "Your mother wants your homework done before you watch television."
* Realize that as a step-parent, you can play a special role as a mentor and friend. But don't try to replace the biological parent or put the child in the middle of parental quarrels. Even if the biological parent is dead, he or she still holds an emotional place. It's important to say, "I can't be your dad, but I care about you. I am here for you as your step-dad, if you need me; and I want to be your friend."
http://www.theparentreport.com/resources/topics/family_life/teen/381.html
http://www.flc.org/hfl/parenting/stepparents.htm
Strategies for Step Parents
by Gigi Cook
If parenting is the hardest job in the world then step parenting must be close to impossible! Statistics show that half of all Americans will be in a step relationship at one point in their lifetime. What is the secret to great step relationships? Many times putting simple daily principles into practice over time will produce the results you are looking for. Here are 10 tips to help you become a successful step parent:
1. Understand the differences between step families and first families.
Step families are formed out of a loss from death or divorce, resulting in the dissolution of the first family. Children, at any age, may assume their step family will be a re-creation of their first family, often resulting in expectations impossible to fulfill. The key to re-defining "family" is to develop an identity as a group. Hobbies and interests encourage bonding. Whether it is rock collecting, traveling, or Sunday get-togethers, develop a unique identity for your group.
2. Don't expect an instant relationship.
While you cannot expect instant attachment or love, you can expect to be treated respectfully. Model respectful behavior towards your step children and let them see you set the example. Encourage trust by never making negative comments about the biological parent or siblings around your step children.
3. Discipline by the biology book.
When it comes to discipline for step children, biology is the key!! Once you and your spouse have set the rules in your home, let the biological parent take the lead, especially with older children. If the biological parent is absent and the step parent must discipline, take the position of "adult in charge," not parent.
4. Get the "You're Not My Parent" conversation out of the way quick!
No matter the age, this topic is inevitable. When it happens, be confident and clear about who you ARE. You may consider actually saying: "You're right, I'm not your parent. You have a mom and a dad and I do not intend to replace them." If the child has overstepped one of the rules and the biological parent is unavailable to handle the situation, you may need to add, "At the moment, I am the adult in charge. Here is the rule you are expected to keep in this house."
5. Keep a united front up front.
When problems arise the kids need to see you and your spouse in agreement. Develop an expected code of behavior for the entire household that applies to ALL children present. If you and your spouse are having a disagreement over discipline, go for a walk and air your differences. But when the kids are watching…you two are one!!
6. Carve out one-on-one time with each child.
While creating a group identity is key to becoming a family, one-on-one time is key for developing a good relationship with your step child. Find time to spend alone together. Where possible, get yourself into their world. Consider volunteering on your step child's sports team or rearrange your schedule to drive to or from school. Drive time in the car can be a good time to communicate.
7. Avoid creating competition.
Your biological children crave your undivided attention and it is important to spend special time with them. Help your step children to avoid feeling left out by keeping your special times with biological children low key. Don't make a big deal out of what you do when the step kids are away.
8. Make a daily attitude check.
When it comes to your step kids, become the poster person for a positive attitude!! Every day, make a conscious effort to forgive the small hurts that eventually add up to a big grudge. No matter how stressful the situation gets, never resort to criticism or sarcasm when communicating with your step children.
9. Avoid the split personality approach to step parenting!
Scheduling step life can be brutal. Two days here, one night there; the whole family can end up with a spilt personality! Remember, your kids may be important members of two households. When schedules clash, put the child's needs first even if it means extra driving, inconvenient timing, or a missed opportunity for you. Your child will remember your cooperative attitude more than anything else.
10. Keep the success of your marriage in focus.
The most vulnerable relationship in the house is your marriage relationship. It may feel selfish at times, but do whatever it takes to keep your marriage in good shape. Statistics show one of the main causes for divorce in step families is the stress of step parenting. Set aside "alone time" with your spouse and guard that time carefully.