In some cases, step - kids can be very manipulative. I knew of a case where the step - kid would pretend to be sick or injured whenever they didn't get their way. They would make the step - parent look bad in front of the biological parent by saying things like the step - parent didn't take care of them properly when they were 'ill'. It was a nightmare for the step - parent as they were trying their best but being constantly undermined.
There was a situation where a step - kid was extremely jealous of the new family setup. So, they started to secretly damage the step - parent's personal belongings. They would break small items like jewelry or office supplies. This was really a horror for the step - parent as they felt constantly under attack in their own home. They didn't know how to deal with this malicious behavior without making the situation worse.