Change Sort order of property on relationship?

I have created a relationship between ItemType A and B. However, I also want to add an additional property on the relationship ItemType. I open up the Relationship ItemType and add a new property. I also set the Sort Order of this property. But this new added property will always turn up to the far right on the Relationship tab. The related ItemType properties will be shown before my new Property. Any suggestions how to achieve this?