It's sometimes hard to know when to step in and when to step back as a step-parent.
Many experts recognize that the more successful step-parents approach the situation like a favorite aunt/uncle, coach, or teacher; someone with limited authority and influence but who can provide instruction and guidance. "My house, my rules" doesn't typically go over very well with children who may still be be coping with the loss of their primary family unit. Young men are particularly resistant to the "new man" in the house. Girls can sometimes compete with a new wife for their father's attention.
If you want to establish a bond with your step-children (and I assume that you do if you married someone with children), now is not the time to "set an example" or establish new rules that are unfamiliar to the children. Be accepting and supportive of the children and your spouse (the bio-parent). Honor the relationships that exist. Do not try to replace a parent who isn't absent, even if you think the other bio-parent isn't doing a very good job.
Some factors that may impact a step-parent's relationship with their step-children are:
How old was the child when you entered their life?
Is the bio-mom/bio-dad still in the picture?
Do the bio-parents (your spouse and the other bio-parent) co-parent agreeably or is it a higher conflict situation?
Are there new siblings in the picture? Are they bio-children of both of you or are they from another relationship?
How long was your new spouse single before you entered the picture?
Did you know that it takes the average step-family SEVEN YEARS to integrate sufficiently to experience intimacy and authenticity in step-relationships? (Becoming a Stepfamily by Patricia Papernow)
A general rule of thumb is however old the children are when you come into their lives is how many years it will take children to bond with a step-parent.
If you think that there are deficiencies in your new spouse's (the bio-parent's) parenting and disciplinary strategies and techniques, you should address that with your new spouse. Most disciplining should come from the bio-parent (your spouse).
Step-parenting tips and pointers:
Be realistic! Just because you love your new spouse, doesn't mean your kids do (yet)!
Take your cues from the children. If they welcome affection, give it, but don't force it. If they want space, respect their wishes and give it.
If they follow your guidance, continue to assert your authority. If they challenge your authority, back off and deal with it through the biological parent.
If the other bio-parent is in the picture, do not expect the children to call you "mom" or "dad". Think of a nickname only your step-children call you.
Be patient! Don't give up!