In the event of Tsarevich Alexei's untimely death, the brother of Tsar Nicholas, Michael, would become the heir to the Russian throne. In fact, due to the Tsarevich's condition, Tsar Nicholas chose to abdicate the throne on Alexei's behalf, therefore making Michael the next in line. Michael actually chose not to accept the crown unless offered it by a legitimately elected government.

Michael wrote about his choice stating: "Inspired, in common with the whole people, by the belief that the welfare of our country must be set above everything else, I have taken the firm decision to assume the supreme power only if and when our great people, having elected by universal suffrage a Constituent Assembly to determine the form of government and lay down the fundamental law of the new Russian State, invest me with such power."

Michael was obviously never offered the throne because the Provisional Government never held real power. In 1918, he was imprisoned by the Soviets and later murdered.

Source: A People's Tragedy - Orlando Figes