Excellent question, anon, thank you. First of all, I’ll give the proof of Sergei (and Sanya) being Tatar:
The Tatar people are a Northern Asian ethnic and cultural group whose biggest population is found in the Russian Federation, which Tatarstan is a part of. There are actually a few Tatar subgroups. The main three groups are Volga Tatars, Crimean Tatars, and Siberian Tatars. Volga Tatars have the largest population, and are considered native to the Volga-Ural region of European Russia. Judging from the amount of people and their diaspora spread, the Kazarins are most likely to be Volga, though a subgroup has never actually been specified by Ferry.
Tatars have a pretty rich history. Their origins can be traced back to the times of the Golden Horde. Most Tatars who live in Kazan, the capital of Tatarstan, are Muslim. The official languages of Tatarstan are both Russian and Tatar. Tatar is a Turkic language traditionally written in an Arabic script. While many Tatarstani Tatars speak both, Diaspora Tatars usually speak only Russian, or at least, it’s their first and native language. Ferry has spoken about how speaking languages besides Russian is discouraged in the City, so it’s likely that Sergei was never taught Tatar.
Tatarophobia is the term for anti-Tatar prejudice. While it’s generally considered to be the most severe towards Crimean Tatars, it’s still pretty bad towards other subgroups. While the Volga Tatars were historically treated as a “model minority”, receiving better treatment than the Crimean Tatars, prejudice against them is still alive. In 2007, a young Tatar man was stabbed to death by a group of people on his way to work in St. Petersburg. After Elmira Abdrazakova, an ethnic Tatar, was crowned Miss Russia in 2013, she was bombarded with racial slurs.
Being Tatar adds a great deal to the Kazarins’ characters, in my opinion. I think the idea of Sergei being a diaspora Tatar seperated from his ethnic culture does a great deal to explain why he’s so tight-fisted over Sanya — losing his family hits a lot harder without real ancestral or cultural ties to lean on. I think them being Volga in specific adds a fun thematic link, with the Volga being a “model minority” while Sanya, with her albinism, could perhaps be considered a “model mutant”. Another fun tidbit about Tatars in history is that many noble families in the Tsardom of Russia and Russian Empire had Tatar origins. I think that this therefore creates another fun thematic link to Sergei being probably the most empowered member of the main cast. And, as I’ve said before, Sergei claiming that “this world is a cruel place to people like you” to Sanya hits very differently when it’s taken to be not only about prejucide against stalkers or mutants, but racism as well.
Getting into some personal headcanons: I believe that Sergei and Sanya are second-generation immigrants. Viktor Kazarin grew up in Kazan, but him and his father moved to the City after the death of his mother. Grandpappy Kazarin faced a lot of Tatarophobia and Islamophobia as an openly Muslim Tatar man in the majority-white, majority-Orthodox City. This encouraged Viktor to distance himself from his culture. He married a Russian woman ans never taught his children Tatar. I wrote a fic about all this if you’re interested.
Overall, the Kazarins being Tatar is just a really good part of their characters that adds a lot of depth and interesting points. I recommend that you do your own research into the Tatar people and Tatarstan so that you can come up with your own ideas and understanding.