What does Sibēria mean in Latin?

Sibēria, a proper noun, means “Siberia (the region of Russia in Asia)”.

What is the genitive of Sibēria?

The genitive singular is Sibēriae. Sibēria is feminine, first declension.

In classical Latin, [sɪˈbeː.ri.a]; in ecclesiastical (church) Latin, [siˈbɛː.ri.a].

Russian Сиби́рь (Sibírʹ) → Latin Sibēria.

proper noun · femininefirst declension

Sibēria f sg (genitive Sibēriae)

Classical[sɪˈbeː.ri.a]Ecclesiastical[siˈbɛː.ri.a]

New LatinSiberia (the region of Russia in Asia)
Nam im auctus eſt iſte titulus, ut praeſcripto Magni Tzaris et magni Ducis nomine, poſt qvaedam verba initium tituli abſolventia Moſchorum Imperator qvoque appelletur Tzar Caſani, et Tzar Aſtracham, Tzar Siberiae, ac demum non procul fine, Carthalinicorum et Gruſinicorum Tzarium dominus atqve dominator.For this title has been expanded, so that when the name of the Great Tsar and Grand Duke has been given, after certain words completing the beginning of the title, the Emperor of the Moscovites is also called Tsar of Kazan, and Tsar of Astrakhan, Tsar of Siberia, and finally, not far from the end, lord and ruler of the Tsars of the Carthalinians and the Gruzinians.1599, Conradus Samuel Schurzfleischius, “Steph. Ratz. Cosacus”, in Opera Historica Politica, […], Berollini [Berlin]: […] Heinr. Joh. Meieri, →OCLC, page 722:

Latin -ia Latin Sibēria From Russian Сиби́рь (Sibírʹ) + Latin -ia.