Fighting!!

Okay - My kids are driving me crazy!! They have started fighting. I don't mean the regular sibling rivalry stuff I mean knock down drag out fighting. They stand nose to nose and just scream at each other and nothing I do seems to help. I've put them in time out and then Ethan starts saying things under his breath, "Thanks a lot Holly", etc. They end up yelling across the room from each other. To top it off I caught Ethan trying to throw a block at his sister and I caught Holly walking up and spitting on Ethan. They chase each other and knock each other down, everything.

They play great together one minute, all hugs, they share and all is well. Then the next one of them gets the idea it might be fun to spit on the other. Holly hates to be without her brother too. She won't go to bed without him. When he gets into trouble she will sit with him in time out and put her arm around him to make him feel better. I just don't get it?

I've been really careful to make sure it is not one or the other of them starting things, because I know when my brothers were little that Ryan would always get Jeff into trouble by starting something, but it is both of my kids starting the fight.

I don't know where they are getting this from because Ryan and I don't fight like that, if anything we give each other the silent treatment. We don't yell at our kids except to be heard over their fighting... I just don't know where this is coming from. They even yell at each other at bed time. Ethan will start singing and Holly will yell even scream at him to stop.

Does anyone have any ideas? Thanks for any advice...