Oh, mine have been bickering more lately too!  The heat?  I don't know...

I try to talk to them on their level about whatever the issue is (like screaming at the top of your lungs does not get you want you want...)  But I seem to have to repeat myself every day.  Several times a day.

And, yes, I've heard they should start working some problems out themselves, but here it tends to fall into a physical fight within a minute or two...  Tempting as it is sometimes, I just can't let them beat on each other. 

Divide and conquer. Sometimes I just have to make them go in separate rooms or just set them up with different activities.

My 5 & 7 year olds are best friends... sometimes I tell them that they will be put on "sister restriction" and not be able to play together. That usually works! lol

My kids go from joking to flat out furious in about 3 seconds. I make them sit on the sofa and hug one another until I say stop. They start out angry about it but usually end up giggling together or planning the next thing they will do by the time I tell them to stop

MINE TOO!!  I don't get it... all of a sudden they are in complete competition with one another and argue about everything!  It is driving me nuts.  Mostly I have been repeatng myself constantly. grrrrr....

I try to stay out of bickering as much as possible. They need to learn to work it out on their own. If it gets too heated they get separated or sometimes forced together- joined at the hip sort of- until they can get along.

Mine don't get physical often- or ever really-but if it did I would sit them across the table from each other and make them talk it out. I did mediation with inner city kids in college like that. To give them a barrier but they still had to talk it out.