What's Our Relationship?

In one world, she exists, but in another world, another pony exists in her place. Just because her parents loved somepony else. They are children of the Mane Six, but they don't share the same world and yet they share the same parents, or at least one of them. Are they siblings? Step siblings? Cousins? What do they call each other? The one thing they do know is they need each other