My husband and I have been married for about 18 months. We each have a daughter from previous relationships and we also have OUR son. My daughter is 6, his is 4 and neither of them seems to understand that we are happy and we want them to be as well. Instead we have two little monsters who are constantly fighting with us and each other. My daughter sticks to trying to be the victim and turn me against my husband and stepdaughter and his daughter takes the more subtle route of making snide comments when Daddy's not looking. The boy, however, is wonderful. This brings me to the conclusion that the difference is either biology or sex. Since I've done my share of babysitting, I can tell you that the behavior is not isolated to girls, therefor, these kids I think can just sense that they are not made of us, so they are fighting against their respective step-parents. On top of that, my daughter never met or had contact with her biological father, in fact, my husband is the only person she has ever referred to as "Dad". So she doesn't even really have an outside influence from a jealous ex! But his daughter only spends the weekends with us and is with her mother the rest of the time. Oh and does "mom" ever love to tell kiddo that stepmother is the devil! All I can say is, enjoy the time you all get with your spouse, be understanding of their frustrations, and communicate your concerns, even if all that does is piss them off! My husband and I have the "joy" of dealing with this together and understanding the problems with the girls all too well. We know two things, we can't change the way they feel, and we only have to deal with them for 12 and 14 more years! Hope this helps anyone out there. Take the kids to the Grandma's for the weekend and enjoy some grown-up time as often as possible, too!!!