Can one kid really throw everything off balance???

Our son is turning two this week.  Up until now, he has played awesome with his cousins on dh's side.  (five year old boy, three year old girl, and another girl his age.)  Family gatherings were so enjoyable because they all get along so well.  Of course, you always have a couple minor squabbles over toys etc, but overall it was so fun to watch all of them.

Well, bil just got married to a girl who has a three year old from a previous relationship.  He is a terror!  I'm not talking about normal toddler aggression - has no structure, no parenting, no rules, just an awful situation. 

Over the weekend, we all got together and the kids got to see one another.  Oh my gosh, the dynamic has completely changed!!!!  All this kid does is scream at the top of his lungs and run through the house.  Of course, he got all of the other kids wound up.  Before you knew it, there was hair pulling, shoving, screaming matches, etc.  Can one kid really throw everything off balance?  It was so diappointing because everytime this kid did something, his mom did nothing!