when you marry a woman with older children do you marry them too????

Alot of us want to do the right thing by excepting the whole package when we marry into a large family.But why is it that your spouses child or children tend to get themselves involved in your personal business when you or the other parent have a dispute?Situations like this normally turn out bad and when this happens someone gets hurt physically,mentally,and emotionally..WHAT DO WE DO????