As a brother-sister team, they've been used to each other's character, personality, appearance for their lifetime, it would be very difficult to adjust to another partner. As it was written before, ice dancing is more than romance and lust on ice, we've meanwhile witnessed quite some successful siblings teams in this sport. I really find it annoying when these well-matched teams get constantly accused of being incestous. That's simply offensive. The Shibutanis look so nice together, they match perfectly, as other brother-sister teams did/do, and if they like skating together, they should be given any support to pursue their carreer together. As with any couple, it only works as long as both partners want the same. In their case, it seems like they work very well together.