Obtaining and holding on to Sakhalin up to WW2 is doable enough. Beyond that, you'd need the Americans to get in there first at the end of WW2 to preempt the Soviet annexation, which is a pretty tall order.
To be fair, that could require the Soviets to be too exhausted to continue in WW2, to the point where the Soviets can't declare war on the Japanese later on, when they did.
Although, for the northern half... If the Soviets are less successful initially during the revolution, and the various bits of Russia continue to exist (continuing Far Eastern Republic), the Japanese might be willing to offer the Far Eastern Republic a bid to buy the northern half of the island, along with settling any other boundary issues (along the Korean border) in exchange for both monetary support and recognition. The deal could extend to other areas as well.
Or perhaps some timeline where China manage to maintain Sakhalin and the Japanese still engage in a war against China that results the Japanese winning and, instead of just Formosa, they claim Sakhalin as well? Or even have the original Russo-Japanese treaty about joint settlement sans actual delineation remain the treaty in force, instead of the Japanese trading away their claim to Sakhalin for the Kurils. Assuming everything remains the same, in a later Russo-Japanese war, they can claim that since they already possess half of the island de facto, the only way that any territorial claim could be resolved would be to claim the north. (along with at least half the Kurils).