And Don't expect it to be instantly all roses. Do let the biological parent discipline. The children may resent the stepparent's mere presence. Therefore, don't add fuel to the fire by having the stepparent responsible for the disciplining. The rose only flutters in the eyes of the biological parent, but breaks in the step-relationship.
Adolescence is a difficult state which affects all family members.
I t is compounded in step-families because of the other dynamics involved. Do discipline your teenage child. Don't give up on them. Do go with the flow. Flexibility is a great asset especially in the sensitive step-relationship.