Stealing is a serious offense.  Have an honest discussion with your step daughter about her acts.  Try to uncover her motivation for stealing.  Does she need the things that she steals?  Does she want them?  Does she do it for the thrill of it?  Does she do it for attention?

If she needs the things that she steals or wants more attention, that can be easily corrected.  If the want is her driving force, make her write a list of things that she wants and as a family decide how she can earn these things.  For example, you can give her some chores and if she completes them as scheduled and without whining for a period of time that you mutually agree on, she can get that reward.  If she steals because it feels good, you may want to involve a therapist to help you get her back on tract.

Whatever your daughter's motivation, stealing is wrong.  You must convey that message to her.  As punishment, try suspending her privileges.

Bets regards.