If you want to try and combine when your sister registers, try having her link it to your my.avast.com account. That way it would mean you could monitor the status of her installation. Though come re-registration time it would still need to be registered again.

Me I wonder what the true benefit of the my.avast.com account is if it isn’t smart enough to be able to combing all registrations to the same date. Plus give the ability to re-register all linked accounts through the my.avast.com account at the same time.