[lentulusgracchus]

The Supreme Court plays no real role in the ratification process.

Or in the admission of States to the Union, or in secession.

[Non-Sequitur]

Or in the admission of States to the Union, or in secession.

Admission? No. But once a state has been allowed to join then the Supreme Court has original jurisdiction over all cases in where a state may be a party. That includes illegal attempts at secession.