Stalin's son Vasily did survive. Having him succeed Stalin would require some major handwavium, especially since Stalin himself (rightly) saw Vasily as a drunken, incompetent, spoiled brat. He was never groomed for leadership and was actually convinced he would be liquidated after Stalin died.
Besides, the USSR by this point is far too anti-monarchist and (hypocritically) anti-imperialist. So even if you have one family ruling it will never be an official empire. That's why I suggested a really early POD, when things had a greater chance for fluctuation/change, etc.