All of the information which I have gotten for Tenchi's and Tennyo's older half sister Miyuki Kuramitsu mostly comes from the Wikipedia article on Tenchi Masaki. There is a section which focus on Tenchi's family. Miyuki is listed as Tenchi's older half sister from a previous relationship of Kiyone's.
Masaki K. had stated in an interview that Kiyone had an affair with a male member of the Kuramitsu family before she met Nobuyuki. Kiyone's affair with her boyfriend was opposed by the Kuramitsu family because Kiyone was from Earth.
Kiyone became pregnant before she ended the affair with her boyfriend. She may have place the fetus in stasis like her mother did after she became pregnant with Minaho. After Kiyone married Nobuyuki, she had decided to remove the fetus from stasis, and carry the pregnancy rest of term
There is likely a chance that Katsuhito and Airi had intervene during the time of Kiyone's afffair. Miyuki's own biological father may have suggested to Kiyone to name their daughter Miyuki after the other Miyuki Kuramitsu. Miyuki could have been adopted into the Kuramitsu family by her biological father.
I will speculate that Kiyone had the affair with Mihoshi's own father Nakita Kuramitsu. Nakita may have brought Nobuyuki and Kiyone together since Nobuyuki was a friend of his.
About the way which Miyuki will look. She can look like Kiyone Makubi from the Tenchi Universe series. Mihoshi and her brother Misao look more like their mother.
