Sibling Rivalry

This is new territory for me.  I find myself torn between stepping in to teach and letting them work it out for themselves.   How do you deal with sibling rivalry in your house and why do you do it that way? 

Costume Conversation

Alexis-  I don't want to be a bunny for Halloween, I want to be a LION.
Jenn-  We don't have a Lion costume, you can be a Lion next year

Pause

Alexis-  HEY!!  I'm thinking about we could buy a Lion costume.  I want to be a Lion for Christmas.  I want to be a Christmas Lion.