Theory about Lenya Polivanov
Anastasia will go in Sasha's place on the Venus mission (this will not end well) and I think Valya will get found out and sent to the gulag or flat out executed by Raskova (or Irina for the plot), leaving Sasha and Tanya to end up together.
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Besides, I suspect Tanya is already pregnant.
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In episode 1 when Sasha and Tanya are having sex there's a brief throwaway line about him not pulling out...i think that was setting up this to come true.
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Idk, I could be wrong because I dont go and look at these characters' Wikipedia pages but to my knowledge the only person we know for sure is Lenya Palivonov's parent is Sasha...we havent seen his mother or heard about her alluded in some way to my knowledge