Is it wrong for me to leave the state for a week because i got in a major fight?

I don't think you are wrong. But how do you ever see working through this with your sister if you aren't there? It may be the easy thing to do and give time for people to calm down. However, if you don't work it out, you will probably regret it later on down the road. So could you go and make sure you come back before your sister leaves to you can work things out as best as can be? And i'm sorry about your sister and nephew - I'm sure that's very tough.