Even if you discount Ushimaru’s silhouette, any time the Shimotsuki name has been mentioned, Zoro is usually involved.
But we know how and why already right?
Shimotsuki Kouzaburou - Koushiro - Kuina - Wado Ichimonji - Enma
Shimotsuki Yasuie - Toko - Hiyori - Enma
Shimotsuki Ushimaru - Onimaru - Gyukimaru- Shusui - Ryuma
Done.
That's a closed loop for all Shimotsuki clan members and they all involve Zoro and that's fine but thats all there is to it.
These are closed stories which do not require Zoro to be related to anyone to remain closed stories
The open plot point in any of these is the black blade stuff and again that has nothing to do with Zoro being related to anyone