R.K.
Oh J.!
I thought I was the only one with this problem. My situation is a little different however the same. I have an 11 year old step-daughter and a 4 year old son. My step-daughter comes over a lot. Every time my son and step-daughter get together they fight. I like you thought that everything would be fine and they would get along. I was SO wrong!
So... here's what I did and it seemed to work. I wrote down some simple rules (no hitting, punching, disrespect, slamming doors, etc) and explained to them that each time I had to intervene the following would occur:
For the 11 year old: she would kneel in the corner with her hands above her head for 11 minutes
For the 4 year old: he will stand in the corner for 4 minutes
*** my punishment for the 11 year old is harsher because she is the older child and should be setting the correct example.
This worked great in the morning however in the afternoon they were both tired. When they both started giggling in the corner I was forced to add on to the punishment. Therefore I made them run around our house once for each year of life (4 times for my son and 11 times for my step-daughter). Believe it or not, that's all it took and they were great for the rest of the evening.
Day 2... they didn't go in the corner once.
Now, I don't expect the children to be perfect all the time. I only intervened when I thought their disagreements were going too far.
Good Luck!! I hope this helps!!