sorry. all I could do was picture Vader saying .. sister.. and the twin sister thing.......why?
even as a joke post its not relatable at all...
In reality .. the difference would be he has a sister, so long as Willi is alive and reproduces its no issue.
If he died prior to children, there are lots of other people in line or the queen could rewrite the rules or at least attempt. I agree with @deevy post above