I have no problem with silly "opposites attract" cliches and hijinks that could ensue trying to blend two families with 18 kids! Lots of funny misunderstandings and problems and disasters could occur naturally. But these kids are cruel and absolutely unlikable. You're telling me that Mr Shipshape Admiral wouldn't teach his kids to "do the right thing" and Miss Liberal Artsy-Fartsy LoveMama wouldn't teach her kids empathy and kindness?
No, the oldest boy who wants to be class president and get into an ivy league college and seems totally straight arrow is going to play mean tricks, and the oldest girl is going to deliberately steal the guy her new sister likes just to be cruel? The kids are going to totally wreck the house and feel no remorse? Then they are going to deliberately try to ruin their parents' marriage with no care or concern for the pain they will cause parents they supposedly love?
I can overlook the absurdity of two people with so many kids not even talking about where they will live or how they will discipline the kids or what they will do to help the kids get along before they suddenly marry...but then that would be the first thing they'd discuss after the wedding, right? Those are important issues likely to need some problem-solving and attention. "I'm sorry, Honey, I just can't live with a pig in the house. Can we build him a place in the backyard?" "Okay, but I can't accept spanking. We need to talk out problems with the kids." "Okay, well I need some rules and schedules..." and so on.
Even knowing the kids would learn to like each other in the course of the movie and start standing up for one another, their behavior in the beggining was so mean and hateful and bratty that I found them all unlikable characters. The humor just wasn't funny, either. Okay, the ridiculous one scene with the chain of kids dangling out the window was kinda funny...and it had nothing to do with being cruel or trying to hurt someone else for spite...the whole movie could have been honest misunderstandings and disasters that could happen in a houseful of kids of all ages. Instead, it was unbelievable, predictable, and mean. Tossing a cherry on top of a pile of cow droppings doesn't make the droppings suddenly sweet and delicious.
This movie has a few moments that could have been good if they weren't sitting on top of all the rest of it.