I really don’t think any fault can be put on the child, especially when discussing kids younger than preteens or teens, which is a whole other set of issues. If your stepkid is a monster, it’s probably because they aren’t being parented. Kids want to feel like they belong and that they are loved by the people entrusted with their care. If the biological parent is firm in setting up rules, so many of these issues just wouldn’t take place. In a family we all speak kindly and respect each other. We all listen to each other and care about each other’s feelings. If we don’t follow these rules, there are reprocussions, whether it be a time-out or a favorite toy taken away. Maybe I just never had huge issues with my stepson because my husband always enforced these rules with me, no matter which kid we were parenting. And sure, the full-time custodial parent may be a total evil person who tells their child they can act like a jerk at your house, but kids are magical. They are able to follow different rules at different places, which is why the majority of our kids don’t go to school wearing pajamas or why they don’t throw a baseball when at a doctor’s appointment. Once again, it’s your partner’s job to discuss this with their ex. If your partner isn’t communicating with their ex than your partner isn’t being a good parent.
Parenting can be hard, whether parenting kids you gave birth to or parenting stepkids. I’m pretty positive more than half the battle is making sure you and your partner agree on how you want to parent. At times it hasn’t been easy, at times I had to take my husband aside and voice concerns I had over my role as step-mother, but through the years, the most difficult part for me has been not being able to see my stepson every day. Any issues I had with him weren’t his fault, but the fault of miscommunication between me and his father.