1. Ushimaru's not Zori's father.
2. Zori, Sanji, and his siblings were born in the same year, meaning Ushimaru impregnated Sora immediately after Sanji's birth, and Zoro must've been born within 7 months or less. And all of this is when Kaido's takeover of Wano was happening.
I know it's not so common, but people should at least use commonsense once in a while.