What? Kozaburo only had son and that is koushiro. Am i missing?
It makes perfect sense if you think about it from Kuina's story. If Kozaburo had a son and a daughter, then who stands to inherit the Dojo, and the Shimotsuki style of Swordsmanship? The Wano tradition is that Women can't become strong Swordsmen, and that is how Kuina viewed the World. If Zoro's mom was Kozaburos daughter, then that means Koshiro was the one who inherited the Dojo, making Zoro a member of the Branch family.
Way more of a believable scenario, then Ushimaru being Zoro's dad, when Zoro was born in East Blue, and Ushimaru lived in Wano where not such surname of Roronoa exists to our knowledge.