Use the Time, Energy and Money grid described in Jeannette Lofas' book Stepparenting to structure the household. All members will know their role and duties in the family. There will be rewards for completing chores and consequences for not competing duties. All are spelled out and known to family members. Clearly sort out discipline and guidance methods and styles as a couple.
Couples decide on discipline and bio-parent generally directs behavior.
In the absence of the bio-parent, the stepparent reminds the child of household rules. An effective parent or stepparent disciplines the action and the behaviors and does not put down the child, thereby keeping the child's self-esteem intact. Don't take kid's negative behavior as a personal insult. Speak to the children about feelings, fears and concerns. When you notice 'acting out behavior' the need to act out diminishes in direct proportion to the child's feelings of being acknowledged.