Sounds like the royalties 1099 has one SSN (or tax ID#) and that entity/person must file that income.
If it comes to one of the persons, how you work it out with your sibling is up to you. If I got the royalty and paid half to my sibling I think I would work out that my sibling pays some tax back to me.
If it comes to a trust or other entity that is not one of the people, then that entity has to account to the IRS for any income and distributions.
