It was previously stated in the manga that Hashirama Senju and Mito Uzumaki had a child together, but the manga never stated what happened to him. I believe that Hasirama's son for some reason moved away from the village, had a child himself, and gave birth to Nagato. In the latest manga it was stated that Nagato was from the Senju lineage and we already knew he was an Uzumaki.
If Hashirama really did only have one son, this means that Nagato and Tsunade are brother and sister. But if Hashirama had more children besides the one with Mito, then that could make Tsunade Nagato's cousin or a very close relative.
You must be registered for see links
If Hashirama really did only have one son, this means that Nagato and Tsunade are brother and sister. But if Hashirama had more children besides the one with Mito, then that could make Tsunade Nagato's cousin or a very close relative.