Is it permissible for the cousin to return to Russia to live with his wife and son, or should he leave them there to please his parents?

Obeying parents is obligatory unless they command disobedience. Leaving one's children in a land of disbelief is a sin and a betrayal of trust, especially if their mother is a disbeliever. The man must return to his children, raise them, and provide for them. He is not permitted to obey his parents in this act of disobedience. He should strive to please his parents and work to bring his children back to his country, even if by stratagem, because a disbelieving wife has no right to custody. Similarly, if a Muslim wife insists on not traveling with her husband to a distant country where he intends to reside, then custody belongs to the father.