Take one dose of humor

Photo by Denis L. Pigostin; source: www.stockvault.net

As the familiar quotation reminds us, "Laughter is the best medicine."
Life as a step family doesn't bear much resemblance to the Brady Bunch. Problems aren't solved in a half-hour slice of life and children rarely give a step parent an eager welcome to the scene - no matter what ages the children are.
Nor is it easy to parent someone else's child. The step parent usually doesn't know the child from the earliest days, when the child's personality is shaped. Other people influenced the child at that time.
Typically, there is another bio-parent whose words and actions have direct effect on the child, be it positive or negative.
In moments when a resentful stepchild mouths off at a step parent, it can be tempting to blow a gasket. After all, you didn't ask for this disrespect - or did you? Your own reactions in those moments has more affect on the situation than you might think.
Stepchildren - no matter what their ages - are still children. They can be counted on to behave like ... well, children.
In that moment, remember not to get drawn down to a childish level yourself. See their behavior for what is really is: Childishness.
The same type of behavior you exhibited when you were a child.
A bit of humor goes far to defuse the situation and keep things from getting out of hand and causing more hurt feelings.
It also helps to remember that in today's world of diverse family structures, you are far from alone. Many others have gone before you or are in the same boat now.
If laughing at the riotous experiences of others is any help, try some books that share the stories of other step parents.
"The Single Girl’s Guide to Marrying a Man, His Kids and His Ex-Wife," by Sally Bjornsen, and "The Package Deal," by Izzy Rose, are humorous looks at growing into the life of step parenting. Both books are available through Barnes and Noble, Borders and The Tattered Cover.