People are saying his brother, Klay Kubiak, but I think it's all speculation and nobody I've seen say that is someone who would have actual sources.
But, the speculation makes some sense. Klay has been the assistant QB coach for the 49ers for two years. Before that he spent one year as a defensive quality control coach with the 49ers. The year before that, he was a head coach and offensive coordinator at a high school in Houston. It would be a step up, but the move would make some sense.