Not a surprise at all that Anna speaks Russian and not Belarussian. When Anna was growing up, "Belarus" was barely a country and was really a satellite territory of the U.S.S.R., it had a far less distinct identity than some of the other Soviet Republics. And in a city like Minsk, you would have, and still will have, several Russian speakers.
More importantly, Minsk was part of the "Pale of Settlement" where Jews were permitted to live under Russian rule. This was not considered Belarus by Jews, but "White Russia." If Anna's family is from that area, it is not because they were native Belarussians who happened to be Jewish, it is because they were Russian Jews who were permitted only to live in certain parts of the Russian empire.