Shimotsuki Kozaburo 55 years saved a village from bandits.
He taught them Swordsmanship, the change the village name to Shimotsuki village.
He found a wife there, and had a child called Shimotuski Koshiro, so Kuina must also be a Shimotsuki.
Zoro's name is Roronoa. We know of Shimotsuki Ryuma, Ushumaru, Kozaburo and Yasuie.
Zoro's name is not Shimotsuki, but he has some Shimotsuki traits.
I'm guessing Zoro's mother is Shimotsuki, and his father is Roronoa. That would explain why he's not Shimotsuki in name but. has similarities.
This village had people settled there before Kozaburo arrived, so there's people that he could have met to marry, and people there to marry his children.
Kozaburo found a wife in East Blue, his son (Koshiro) had a daughter.
Koshiro's daughter (Kuina) is a little older than Zoro meaning Kozaburo potentially had a daughter who married some guy called Roronoa from East Blue.
For Zoro and Kuina to be similar age, we're guessing Koshiro and his sister who must have found partners to marry in the village had kids around the same time.
So, Koshiro might actually be Zoro's uncle, not just his sensei. So, in a sense, he's a Roronoa who has relations and genes from the Shimotsuki clan, and Kuina is his cousin I guess.
Tashigi is likely a cousin, similar age, so quite possible that Kozaburo had multiple children who carry Shimotsuki genes. Zoro looks exactly like Ushima which is no different to Tashigi looking like Kuina; happens that people end up looking identical to relatives from decades of years ago.