Thanks for calling me reasonable, Prettyfly. My ex and his wife usually tend to call me the "unreasonable, sad, vindictive b*tch", which shows that there is always two sides to a story.
Getting back to the family thing ... as hard as it might be for parents and parents' partners, imagine what it must be like for children.
"My house, my rules" ... "You can do this when you are with your mother/father, but at my house ..." ... I'm not even on about badmouthing the other parent, but about devaluating and questioning the child's life per se. Bear in mind that this child probably leads a perfectly happy life with the other parent, no matter what you personally think about his/her parenting skills. The child's life is really a combination of two lives, and nobody likes to hear that parts of their life are actually wrong and rubbish.
As far as I am concerned, my ex and I are separated ... but we are still our daughter's core family, the people who created her and jointly raised her for the first 6.5 years of her life. The way our daughter turned out/will turn out in the future is down to the values, type of education, rules etc. we jointly agreed. My ex has a new wife, and that's that. But my daughter doesn't have a new family, she still has her old one with a couple of extensions built on! After all, SHE didn't separate, did she?
Within this family plus extensions, there obviously need to be boundaries. I have learned over the years to consider our daughter "the school hamster", a project with two team leaders. Sounds crazy, but it takes the emotions out. I will communicate with the other team leader about our project, for "offical" project meetings (like parents' evenings) we are both available. Within reasonable boundaries, each team leader is free to organise his/her team to guarantee the success of our project.
Needless to say, the ex's "Brady Bunch approach" and my "project management approach" do clash.