Oda just revealed Zoro’s family tree in the new SBS Volume 105.
Here’s what he said:
There are many questions regarding Zoro’s bloodline. I don’t think there is any need to elaborate on it further, so I will just announce it here.
And there it is, the secret of Zoro’s lineage is revealed. Grandchild of Pinzoro, child of Arashi, grandnephew of Ushimaru.